Red Crossbills are predicted to move south in significant numbers this year—check out this article to learn fun facts and ID tips for these enigmatic birds. Finch irruptions are often exciting events, enticing birders with crossbills be crunching away in the conifers, and the potential for additional fun like Pine Grosbeaks dripping from the local crabapples. Ron Pittaway’s finch forecast is always a much-anticipated read for US & Canadian birders in the fall, and this year is no different! At least partially due to drought conditions followed by a record wet spring and summer in the Northeast, this year’s cone crop looks to be perhaps a once in couple-decades event. As a result, Red Crossbills will feature prominently in this year’s flight.
When crossbills were on the move in 2012, we were pleased to feature an article here by Matt Young, one of the North America’s experts on this incredibly complicated species complex. This year Tim Spahr is joining the team, who, since that 2012 movement, has become an expert in “typing” crossbill recordings. Tim has made several trips to Arizona (along with Lance Benner) to get a better handle on Type 6, in addition to trips to coastal and intermountain areas of the west to record other call types. He also has been traveling around portions of the northeast this year to help us get a better handle on what types were breeding this past summer.
Thanks to Matt and Tim for this summary, which is a revision of the 2012 article to include the recent (2017) split of Cassia Crossbill (Loxia sinesciurus), the recognition of Type 11 in Middle America, and updated information on the other types. With the loss of Cassia and gain of Type 11, Red Crossbills still have at least ten distinct call types in North America, each with its own key conifer(s), areas of core occurrence, and patterns of movement.
by Matt Young and Tim Spahr
Red Crossbill (Loxia curvirostra) represents an ecological puzzle for biologists and birders alike, and an opportunity for pioneering citizen-science driven fieldwork for those inclined to explore some of North America’s little-birded coniferous habitats. Since Jeff Groth’s landmark work in 1993, the value of recording crossbills for identification to type has become increasingly recognized. Groth’s work laid out the idea that each taxon gives a unique, identifiable call type when in flight. As many as 10 “call types” of Red Crossbill can be found across North America (Groth 1993, Benkman 1999, Irwin 2010), each of which may represent a different incipient species (Parchman et al. 2006). The flight calls given by an individual bird have been confirmed to be relatively stable over time (Sewall 2009, Sewall 2010). These call types have also been shown to correspond with slight differences in morphology, genetics, and ecological associations (Groth 1993, Benkman 1993a, Parchman et al. 2006). Just this year, after several years of painstaking research by Dr. Craig Benkman and his team, “South Hills Type 9”, was elevated to the species level as Cassia Crossbill (Loxia sinesciurus)–named for one of the two counties in Idaho where the species occurs–because of its differences in genetics, ecological association, and vocalizations.
It can be quite challenging to differentiate flight calls of the various Red Crossbill call types, but with some practice it’s not an impossible task. The flight calls are the sound typically described as jip-jip-jip and most frequently heard when the birds are flying overhead. In order to find and identify crossbills, it’s essential to develop a familiarity with their flight call vocalizations, which can also be occasionally given by perched birds. In each section below we will try to describe the differences between the flight call vocalizations of Crossbills of North America, including the various species, and with a focus on differentiating the various call types.
In addition to flight calls, Red Crossbills also give other calls and various songs. Excitement calls, also known as “toop” calls, can aid in identification to call type, but at this time songs cannot be used for identification to call type. In order to stay focused on the best traits to identify birds to call type, we only discuss flight calls in this paper!
To identify Red Crossbills to call type one needs to do an audiospectrographic analysis. Raven Lite can be used to do this analysis. This analysis gives a computer printout of the bird’s voice and therefore represents a signature of the call type. Briefly, this printout shows the frequency of the sound in kilohertz on the “y” axis and time from the beginning of the recording on the “x” axis. It is important to set the limits on the “y” axis to around 10,000 kHz, and have the “x”-axis window (or scale) be around 1 second. When one analyzes crossbill spectrograms, the scale needs to be relatively consistent, and we would emphasize that the larger the scale the better, since using too fine of a scale can lead to missing certain intricacies of a given call type.
DOCUMENTING AND RECORDING CROSSBILL CALLS
We encourage anyone encountering crossbills to attempt audio recordings. While we welcome recordings from those with professional grade recording equipment, even smartphones can adequately document the call types. It is always better to download a sound recording app that makes .WAV files, which prevent loss of important audio information (see this article on making recordings with your smartphone). However, even using the “voice memo” feature can get a decent recording that can help to type the crossbill. For example, on an iPhone just open your audio recording app, hit record, hold your phone as steadily as possible with the speaker facing the crossbill, and then email the recording for analysis along with a link to your eBird checklist! External microphones can be purchased that improve the recording quality even more; check out recommendations from the Macaulay Library.
If you record a Red Crossbill, please enter it as “Red Crossbill” in eBird, upload the recording to your checklist, and send the link to the checklist to Matt or Tim for assistance with identification to specific call type. If identification to Type can be confirmed via the recording, you can easily use the new “Change Species” feature to search for the correct crossbill type and revise the identification. If you try to identify the type yourself, do not worry if you misidentify the proper call type; one of the authors will contact you after listening to your recording. Keep in mind many crossbills can be typed from very poor recordings, so don’t be afraid to submit low-quality media, as these often turn into high-value data.
Below is a thorough summary of the vocalizations of Crossbills of North America, including all four species, with a focus on differentiating the 10 call types of Red Crossbill. Please note that additional Types occur in Europe, Africa, and Asia, and that we welcome insight into that puzzle as well. Please write to either Matt or Tim if you are interested in contributing to future work that addresses crossbills at a global level.
While much has been learned in recent years about Red Crossbills, there is still much to learn. Our understanding of how these populations interact and to what extent they are evolutionarily divergent and reproductively isolated is still not well understood. Birds that give certain call types appear to preferentially mate with others that give their own call type, but the big question remains: is this what happens under all environmental conditions? To answer this question, we really need recordings of paired birds. This could potentially be a great year to get recordings of paired birds in the Northeast under the very interesting environmental conditions of possibly an unlimited food supply.
Some of the statements under “known range” below are still somewhat provisional and reflect only what has been documented to date. The basic ecology of Types 6 and 11 are marginally known, as is the extent of movement that they may or may not undertake. With relatively few recordings from Mexico, there is much complexity yet to be understood, including the very interesting possibility that an additional Type might exist in the hard to get to lowlands of the pine savannas areas of the Mosquitia (pers. comm. John van Dort) of Honduras.
2012 and 2017 IRRUPTIONS
Crossbills were on the move early in 2012, and in July, they appeared on the Farallon Islands off the coast for the first time since 1998 (Alvaro Jaramillo pers. comm.), and these were probably Type 3s. Type 2s were also on the move in July of that year in small numbers into the Plains of Kansas.
Then in mid-August, a very significant west to east Red Crossbill movement occurred across the Great Lakes, Midwest, Ontario, and Northeastern states. This was a large movement and was much earlier than usual. Recordings obtained across the Great Lakes and Northeast during 2012-13 involved primarily the small-billed Type 3, which has its core range in the Pacific Northwest where it is most efficient at feeding on Western Hemlock. The Western Hemlock crop was reported as poor in 2012, which is likely what caused these birds to irrupt southward along the Pacific coast and eastward to the Atlantic coast. Many states, including areas of the Plains, had first state records for Type 3 that year (see map).
Later that winter in January-February Type 10 and other birds that appear to be a good match for Type 7 were both present in numbers during the 2012-13 irruption. Birds were recorded moving north to south along the coasts of Maine, New Hampshire, Massachusetts, Connecticut, and New Jersey with records even coming in from as far as Kentucky, a first state record for either Type 10 or Type 7 (see below for more on this emerging conundrum). Lastly, single Type 5 crossbills were recorded in Kansas, Missouri, Oklahoma, and Arkansas for the first time ever.
Red Crossbills are on the move again this year in 2017, and this year’s movement is shaping up to be even more impressive than the one in 2012-13. Back in June and July large numbers of Red Crossbills (Type 3, based on a few recordings received) were moving down the west coast. Then in July-September recordings of numbers of Types 2, 3 and 4 poured in from Minnesota, Wisconsin, and Michigan. Two states, Wisconsin and Minnesota, had their first state records of both Type 4 and Type 5 (note that Type 5 is an accidental vagrant east of the Rockies).
The breeding season in the northeast this June-August also had larger numbers of birds than usual, with “eastern” Type 10 appearing in numbers in New York, Vermont, New Hampshire, and Maine. Modest numbers of Type 1 were also found in New York and New Hampshire with a sprinkling of Type 2 found in the same states. The native spruce cone crop in these areas is currently the best in a couple decades or more, and is also coinciding with a good to great cone crop on hemlock, white and red pine, and even tamarack – it is rare to see soft-coned conifers such as spruce and hemlocks produce great cone crops the same year as red and white pines.
It will be particularly interesting to see how it all plays out across the country this winter. Expect types 1, 2, 3, 4, 10/7 (see below for more info) and perhaps even a few Type 5 in the Northeast this year, with several types nesting in the area. Type 3 could nest to southern New York/northern Pennsylvania, and Type 1 (and Pine Siskin) could nest south of New York in the Appalachians like they often do. White-winged Crossbill should also spillover out of northern New England and breed locally in small numbers in western Massachusetts and southern New York. In the Great Lakes, several call types (and Pine Siskin and White-winged Crossbill) should linger to nest locally, but the crop is said to be only “good”, not great like it is in the northeast. Will most of the crossbills and other finches move through the Great Lakes is the big question though? As for the other finches, it looks like northern parts of the region might get in on some redpolls (bring them on!), but for more on other finches, see the always fun and informative winter finch forecast by Ron Pittaway: http://jeaniron.ca/2017/wff17.htm
We do expect Red Crossbills to continue to turn up in new areas this fall and winter, and this event should carry right on through into spring. Birds should be breeding in many areas between January-April.
A Call to Action: A few particularly interesting questions with this year’s crossbill movements include:
If you have information or media that might help us answer these questions, please upload any pertinent media (particularly recordings of paired birds) while also adding any useful information in the species notes on your eBird checklist.
RED CROSSBILL CALL TYPES
Type 1 – Appalachian Crossbill (Young et al. 2011) — Medium-billed
Taxonomy: Subspecies might match type specimen for L. c. pusilla, but has also been referred to as L. c. neogaea. This population most likely needs a new name but has yet to be formally described.
Known range: Primarily in the Appalachians from s. New York to Georgia and even Alabama; occasional in Adirondack Mts., NY, and central Massachusetts northward into New England, s. Ontario, Maritimes, and perhaps Great Lakes; rare to very rare in West and rare along coast south of Maine in East. [eBird map]
Movements: Mostly resident in East; rarely irrupts into Pacific Northwest. New records were just confirmed in recent weeks from Montana and Arkansas.
Preferred trees: Red Spruce and White Spruce, Eastern White Pine, and hard-coned pines such as Pitch, Red, Virginia, and Loblolly; in the West has used Sitka Spruce and Western Hemlock.
Flight call: A hard, quick, attenuated chewt-chewt similar to the chip of a Kentucky Warbler; compare to the softer Type 2.
The Type 1 Red Crossbill flight call sounds much like a Type 2 Red Crossbill. In both call types the spectrograms are dominated by a downward component. To be able to identify these two types with certainty, audiospectrographic analysis is essential. The Type 1 spectrogram above is typical, starting with an initial upward component the vast majority of time, and a downward part that descends more quickly than in Type 2 (duration of type 1 call is <.04s). Overall, the Type 1 flight call is a more attenuated, dryer and harder flight call than the Type 2 and it sounds like chewt-chewt-chewt. Like the Type 5, Type 1 can produce sound polyphonically (see Type 5 for more on polyphony), meaning they use separate parts of their syrinx like a Catharus thrush.
Status: West: Type 1 appears to be rare, maybe only occurring in irruption years, in the coastal Sitka Spruce and Western Hemlock forests of the Pacific Northwest (Young et al. 2011). East: The core zone of occurrence (area where a type occurs most commonly) for Type 1 is the Appalachians from southern New York to northern Georgia (Young et al., 2011). In the Appalachians they are most commonly encountered in areas of both red spruce (and other spruces) and Eastern White Pine, and to lesser amounts Eastern hemlock and various hard-coned pines (i.e., Pitch, Red, Virginia and Loblolly pines). The Type 1 Red Crossbill appears to be more of a generalist and is probably the least common of the more widespread call types found in North America.
Type 2 – Ponderosa Pine Crossbill (Benkman 2007) — Large-billed
Taxonomy: Would be most appropriately assigned to subspecies L. c. benti, but, in part, has also been assigned to L. c. bendirei.
Known range: Most common in the Ponderosa Pine forests of the West, but can be found continentwide in U.S. and into very n. Mexico and parts of s. Canada. Walter Szeliga and Lance Benner has been studying the occurrence of this type in s. California, and thus far they’ve added dozens of recordings to the collection from that area (Szeliga et al. 2014). If you get more from that area, please upload them to eBird. [eBird map]
Movements: Highly irruptive and can occur nearly anywhere even into the Plains states. “Kinked” Type 2s have moved in sizable numbers eastward this year, and many records have already come in from the Great Lakes.
Preferred trees: Hard-coned pines. Most efficient at feeding on key conifer Ponderosa Pine in the Intermontane West, but will use other hard pines as well, including Lodgepole and Jeffrey pines (West), Red, Jack, Pitch, Virginia and Table Mountain Pines (East). Uses spruces and soft-coned pines as well. Very eclectic in diet.
Flight call: Like Type 1, but a husky, deeper and lower choowp-choowp or chew-chew; can recall Pygmy Nuthatch or Olive-sided Flycatcher’s pip-pip-pip. Western birds are a bit more “ringing” or even squeaky in quality, as compared to eastern birds. Compare squeaky sounding birds to Type 3.
Type 2 flight calls are a bit lower and more husky sounding than those of Type 1. The downward component of the spectrograms is more gradual and modulated (duration of call is >.04s), and the initial upward component found in Type 1 is absent (unkinked spectrogram made from Macaulay Library #161299). Additionally, the call (as it appears on the spectrogram) will often level out a bit before continuing its downward trend. The call sounds like a choowp-choowp-choowp or chew-chew-chew. Both types can have secondary ending components, but they’re stronger and much more consistently present in Type 1. Additionally, the entire Type 2 flight call is given below 4.5kHz whereas the highest point of the initial upward component of the Type 1 flight call is usually between 4.5-5 kHz. This type tends to produce an “unkinked spectrogram in the east (see unkinked Red Crossbill Type 2 Call). In the west, the Type 2 will often produce what is called a “kinked” spectrogram (see kinked variant above), and birds producing this type of spectrogram seem to be rare in the east. This kinked call type first goes down and then back up before going back down. This spectrogram can look a bit like a Type 3, but the difference in sound is quite evident with Type 2 sounding much stronger, huskier and ringing, and a bit lower in frequency. To the ear, Pieplow (2007) likens some Type 2 calls to the piping calls of Pygmy Nuthatch or the pip-pip-pip call of Olive-sided Flycatcher.
Status: West: More than any type, Type 2 will readily feed on various hard pines (e.g., Ponderosa, Lodgepole, Red, and Jeffrey pines) throughout the United States, but its core zone of occurrence, where it’s most closely associated with key conifer Ponderosa Pine (Benkman 1993a), is the Intermountain West where this conifer is common. East: This type has perhaps the most varied diet of the types and is the most widespread Red Crossbill call type in North America (Groth 1993), even occurring in areas of the Plains where ornamental conifers have been planted. Small numbers of Type 2 can be found every year somewhere in the East, where it often associates with Red, Jack, Pitch, Eastern White, Virginia and Table Mountain pines. Like all other crossbills, it also utilizes soft-coned conifers like spruces.
Type 3 – Western Hemlock Crossbill (Benkman 2007) — Small-billed
Taxonomy: This Type best matches L. c. minor, but that name has also been applied to small-billed Type 10. The nomenclature needs to be clarified in this case as well.
Known range: Primarily in the northern coastal areas of western North America; occasional to Great Lakes into northeast, Ontario and Maritimes. [eBird map]
Movements: Highly irruptive into the Great Lakes, Northeast, Ontario and likely Maritimes every 3-8 years in numbers – last irruption occurred 2012-13 (before that 2006-07), which was one of the largest and earliest starting irruptions for this type on record. Type 3 have moved in sizable numbers eastward in the fall of 2017 as well, and many records have already come in from the Great Lakes and Northeast
Preferred trees: Key conifer is Western Hemlock; uses Engelmann and Sitka spruce, and less often Douglas-fir and Blue Spruce in the West; in the East, most often uses Eastern Hemlock or White and Red spruces.
Flight call: A squeaky or scratchy tik-tik or kyip-kyip; highly distinctive.
Type3_flight_call_ML 136592 (now in ML as ML71525571, despite file names here)
The flight call of the Type 3 is squeakier and more scratchy sounding than those of the other types. Type 3 tends to produce two slightly different spectrograms– one looks a bit like a lightning bolt with its zig-zag appearance – it starts out with a downward component followed by a short upward component connected to a second downward component. The other looks a bit like the letter “n”. Occasionally, there can be tails at both the beginning (less common) and end of the typical zig-zag appearance. Additionally, during the second downward modulated component, the tik-tik-tik or kyip-kyip-kyip call can level out just a bit as it continues downward. Type 3 can sound a bit like a scratchier or squeakier version of a Type 1 or 2, but Type 1 are sharper and Type 2 huskier and deeper sounding. However, the spectrograms of Type 3 cannot be confused with any other type unless too small of scale is used. If too small of a scale is used, Type 3 can look a bit like Type 5 or even a kinked Type 2 — this is a prime example as to why a large enough scale is essential.
Status: West: This call type prefers hemlock more than any of the other call types found in North America. Its core zone of occurrence, where it’s most closely associated with key conifer Western Hemlock (Benkman 1993a), is in the Pacific Northwest. It will rarely occur in spruce/Douglas-fir forests of Intermountain West as well. East: Type 3 is the most highly irruptive type in the East. When Type 3 irrupt east in great numbers (as in 2012), nearly all retreat back to the Pacific Northwest the following May-June when Western Hemlock cone crops start developing again. As with all other Types, Type 3 will readily use various spruces like White, Red and Engelmann spruce, but it appears to most often associate with Eastern Hemlock in the Great Lakes and Northeast.
Type 4 — Douglas-fir Crossbill (Benkman 2007) — Medium-billed
Taxonomy: Unclear but quite possibly L. c. vividior.
Known range: Core area is Pacific Northwest, with birds occasionally found into the Intermontane West; rare in the East. [eBird map]
Movements: Occasionally irrupts to Intermontane West and rarely to the Great Lakes and Northeast. Type 4 have moved in sizable numbers eastward this year, and many records have already come in from the Great Lakes. First state records have already been established in 2017 for Minnesota and Wisconsin and a range extension has also been documented in the Sierra Nevada mountains of California.
Preferred trees: Key conifer is coastal variety of Douglas-fir; less often uses interior variety of Douglas-fir; also uses various spruces and white pines, including when it moves eastward.
Flight call: A bouncy plick-plick-plick or pwit-pwit-pwit; very distinctive, but compare to Type 10 and 6.
The flight call of the Type 4 is one the easiest to recognize even when compared to Type 10 (it was split from Type 10 in 2011; see Type 10 below) and is a very bouncy, almost musical down up plick-plick-plick. The spectrogram (Red Crossbill Type 4 Call) is dominated by a down-up component with the ending section looking very similar to the Type 10 flight call. Sometimes the spec can appear to look like a checkmark as it does in the second spectrogram above. Type 6 is actually a closer match in both sound and spectrogram, but Type 4 always has a deeper “V”-shape in frequency, generally bottoming out around 2 kHz. Type 4’s flatter sound and the lower pitch on average separate it from Type 6’s obvious ringing sound.
Status: West: This type’s core zone of occurrence, where it’s most closely associated with the coastal variety of Douglas-fir (Benkman 1993a), is in the Pacific Northwest. It can also be found occasionally in the Intermountain variety of Douglas-fir in addition to spruces and white pines found in that area. East: Like Types 1, 2, 3 and 10, Type 4 has occurred across North America and is found at least rarely in the East. In the Northeast, it is known from just two recordings from New York and has been found in association with spruce and white pine (and other pines). It should be noted however that there is much dietary overlap in the East across call types and therefore it is not uncommon to find several types feeding on the same conifer species in the same area – this is particularly true in larger invasion years. Other eastern records of Type 4 include a few recordings from Ohio in the 1969-70 (see Borror Laboratory of Bioacoustics’ collection) invasion and a couple from Michigan (Groth 1993).
Type 5 – Lodgepole Pine Crossbill (Benkman 2007) — Large-billed
Taxonomy: Would be most appropriately assigned to subspecies L. c. bendirei, but L. c. benti has also been assigned to birds that may represent Type 5.
Known range: Western in U.S. and Canada; vagrant to the Great Lakes east to at least New York. [eBird map]
Movements: Slightly to moderately irruptive in parts of the Intermontane West. A few Type 5 have already been documented in fall 2017 in Wisconsin and Minnesota, both establishing first state records – will any move into the Northeast?
Preferred trees: Key conifers are Lodgepole Pine and Engelmann Spruce, less often uses Douglas Fir, Blue Spruce or white pines.
Flight call: A springy or twangy clip-clip-clip or chit-chit-chit; quite distinctive and level sounding.
Type 5 Red Crossbills have two elements that drop in frequency, but the two elements are given in very slightly different frequency domains. The lower elements are generally simpler and show less variation individually, whereas the upper elements usually rise sharply before modulating downward (Groth 1993). The second element often starts a fraction of a second after the first element. On the spectrogram this second element sometimes hints at connecting to the first element. Generally speaking, both elements are given nearly simultaneously. The idea that both elements modulate differently, basically over the same time span, is likely evidence that the Type 5 uses different halves of its syrinx, thus producing sound polyphonically not unlike a Catharus thrush (Groth 1993, Pipelow 2007). Unlike other types, the orientation of the call in general is slightly from the top-left to bottom-right as it would read directionally on a piece of paper. To the human ear, Type 5 can sound like very twangy clip-clip-clip and therefore unlike other types except Type 3 (which sounds softer and scratchy).
Status: West: Core zone of occurrence is the Intermountain West. Benkman (1993a) found that Type 5 Red Crossbills were morphologically adapted to most efficiently forage on Lodgepole Pine (Pinus contorta var. latifolia), which produces one of the most stable cone crops in the world (Benkman 1993a). Because of this stable cone crop, Type 5 can be resident throughout much of the west where Lodgepole Pine is common. This type also readily feeds on Engelmann Spruce (Benkman and Miller 1996), and to a much lesser degree Douglas-fir and Blue Spruce (Kelsey 2009). East: It has only been recorded in the Northeast once, from Pharsalia, New York 6 August 2006 (Budney and Young Macaulay Library #138299 (Young 2010). A few Type 5 have already been documented this year in Wisconsin and Minnesota, both establishing first state records – will any move into the Northeast? With recent massive dieoffs of Lodgepole Pine from Mountain Pine Beetle (Dendroctonus ponderosae) outbreaks in the West, this type could become more nomadic and perhaps more regular in the East (Young 2010).
Type 6 – Sierra Madre Crossbill (Benkman 2007) — Large-billed
Taxonomy: Likely equates to L. c. stricklandi
Known range: Southwest U.S. to southern Mexico and Honduras; possibly also Guatemala and El Salvador (recordings needed); in the U.S. it occurs in se. Arizona and sw. New Mexico; museum specimens have been noted from Colorado and California. [eBird map]
Movements: Unknown; some years perhaps more common in Arizona than others?
Preferred trees: Several hard-coned pine species of Mexico, especially Apache Pine.
Flight call: A cheep-cheep, ringing, and tonal. Compare with type 4.
Type 6 flight call aF743 (The Museum of Vertebrate Zoology at Berkeley)
The cheep-cheep-cheep flight calls of Type 6 are tonal and slightly ringing with a downward-modulated frequency and an abrupt terminal rising component (Groth 1993). Type 6 can be confused with Type 4 in some instances, however Type 4’s flatter plick-plick-plick sound separates it from Type 6’s slightly ringing cheep-cheep-cheep sound. Spectrally, Type 4 always has a deeper “V”-shape in frequency, generally bottoming out around 2 kHz. The gentler, “U”-shapes of Type 6 bottom at 2.5-3 kHz. Not much is known about Type 6, but by ear it does sound similar to the large-billed Type 8 of Newfoundland, although the spectrograms are obviously different. Type 6 is much less modulated than the m-shaped Newfoundland Type 8.
Status: West: Type 6 is most associated with large, hard-coned Mexican pine species such as Apache Pine and its core zone of occurrence is the Sierra Madre Occidental (Benkman 2007). Type 6 has been recently documented in southeastern Arizona’s Chiricahua, Huachuca, Pinaleño and Santa Catalina Mountains and the Pinos Altos mountains in southwestern New Mexico. Recordings also document this Type into southern Mexico and further south to Honduras; Type 6 and Type 11 occur sympatrically in the highlands from Chiapas to Honduras. There is one record northward to California (San Diego Natural History Museum SDNHM 873; pers. comm. Lance Benner and Walter Szelinga) and Colorado has six specimens that match L. c. stricklandi (e.g., DMNS 4294, 4296, etc.; Spencer 2009). It has never been found in the East.
Type 7 – Enigmatic Crossbill — Medium-billed to large-billed (Young 2012)
Taxonomy: Subspecies unknown but may actually match the “old northeastern subspecies” L. c. neogaea or possibly ssp. pusilla?
Known range: May be rare, or based on many recordings of “eastern” Type 10 over the last several years, our rather common “northeastern subspecies” or call type. Type 7 was previously only thought to occur in interior areas of the Pacific Northwest U.S. and s. British Columbia, but as stated above, many recordings from the Great Lakes to the Northeast and southern Maritimes, some even going back to 1962, match the spectrograms and sounds of “Enigmatic” Type 7 very well. More study, specifically, recordings and measurements of the same individuals, are needed to crack and solve this mystery with certainty.
Movements: Unknown, but if “eastern” Type 10 is a match, it likely wanders from the southern boreal through the Great Lakes into the Northeast and southern Maritimes; also sporadically in the Pacific Northwest to southern Alaska.
Preferred trees: Unknown, was thought to be possibly Western Larch or Western White Pine, but if above is true, very well could be a bit more of a generalist that uses Red Pine in the East most commonly in March-May when food is at its most limited. Also uses Jack Pine and various spruces in the Great Lakes and Northeast. Likely occasional uses Eastern White Pine and Pitch Pine as well.
Flight call: Husky jit-jit-jit somewhat intermediate in sound between Type 2 and 10, but closer sounding to Type 10. Type 7 is the flight call type that needs the most recordings (and measurements of the same audio-recorded individuals) in order to pin down flight call variability and geographic distribution.
Museum of Vertebrate Zoology, Berkeley Type 7_flight_call_aF497
The spectrogram of Type 7 is often shaped like a small letter “u” or an upsweep with a tail. Type 7 can sometimes sound and look (i.e., the spectrograms) similar to Type 10, but more study is needed! To give you a better handle on what we’re looking into, we present several additional recordings of these Type 10/7 individuals. The main frequency of sound (jit-jit-jit) can be described as sometimes having a short initial fall, or more commonly just having a longer rise followed by a shorter fall (aka “the tail”). Many recordings of ‘eastern’ Type 10, which again may in fact by Type 7, show inverted “v” shapes with a long, descending component to the terminal end of the inverted “v”. Please note that some of Groth’s original Type 7 recordings (such as jM498) are matched perfectly by eastern birds. More recordings are needed. Measurements of a handful of Michigan birds showing spectrograms similar to Type 10/7 appear to be a noticeably larger-billed bird than Type 10s in the west (pers. comm. Jamie Cornelius).
Status: Type 7’s core zone of occurrence was thought to be nw. Oregon, sw. Washington to n. Idaho, w. Montana and s. British Columbia, but the fact that so few birds have ever been turned up in those areas might be better explained by this type actually being more commonly found in the Great Lakes to Northeastearn states, and into eastern Canada. When some of the landmark crossbill studies were first taking place in the 1980’s, very few recordings of crossbills were obtained from the Northeast. In fact, Groth never sampled birds in the northeastern United States or eastern Canada. Benkman did record a few birds in the northeastern states. It is unknown at this time what it most closely associates with, but hopefully additional studies will reveal this soon. If findable, new recordings from Groth’s original Type 7 discovery area are a critical piece to this fascinating puzzle. A team of us are currently studying this call type, please upload any birds you might have recorded!
Type 8 – Newfoundland Crossbill (Griscom 1937) — Large-billed
Taxonomy: The name L. c. percna is usually associated with this type and the subspecies percna is still listed as Endangered by the Canadian Wildlife Service. It has been recently proposed to be down-listed to threatened (http://www.registrelep-sararegistry.gc.ca/species/speciesDetails_e.cfm?sid=814)
Note that some authorities (including eBird/Clements!) use the name L. c. pusilla as a synonym of L. c. percna.
Known range: Mainly restricted to the island of Newfoundland, but also now confirmed from Anticosti Island, Quebec (http://www.registrelep-sararegistry.gc.ca/species/speciesDetails_e.cfm?sid=814).
Movements: Thought to be resident to island of Newfoundland, but at least rarely moves to Anticosti Island (see above report), Quebec; perhaps moves to Magdalen Islands, Quebec, or other nearby Maritime coasts as well.
Preferred trees: Likely associates with Black Spruce.
Flight call: Cheet-cheet, ringing and complexly modulated.
Recent audiospectrographic analysis of recordings during 2005-2011 (n=30; 2 hr 37min of recordings) confirms the presence of a unique Red Crossbill type on the island of Newfoundland (presumably subspecies percna; see taxonomy below). These recent recordings were audiosprectrographically compared to the original two 4-second recordings used by Groth (1993), and much to our surprise, they did not match (Young et al. 2012). The most parsimonious explanation of the differences between the 1981 recordings and the 2005-2011 recordings, then, is that the more recent and complete set of recordings is typical of Type 8 Red Crossbill. Thus we adduce that the more recent recordings refer most reliably to Type 8 (Young et al. 2012). A study by Doug Hynes and Edward Miller further corroborated this finding (Hynes and Miller 2014).
The sound of the original recordings can be described as flat, quick, and a bit harsh compared to the ringing quality of the more recent Newfoundland recordings. The more recent Newfoundland recordings depict a complexly modulated call that vaguely resembles the letter “M” (Young et al 2012).
The main frequency of sound is in the 3.25 to 4.0 kHz range. The flight call can be described as up-down-up-down cheet-cheet-cheet. Additionally, there are often very subtle modulated components attached on either end. Like Type 6, the sound of the flight call of these more recent Newfoundland recordings can be described as bell-like or ringing and clear, resembling the cheep call of the Evening Grosbeak. Type 8 flight calls are much more modulated than Type 6 though.
Status: East: Type 8 is likely most closely associated with Black Spruce found on Newfoundland (Benkman 1993b), but it appears to also feed regularly on White Spruce, Eastern White Pine, and Red Pine and can be commonly found at feeders from February-May (Young et al. 2012). It has declined since the 1970’s, resulting in L. c. percna (Type 8) being listed as Endangered on the Canadian Species at Risk Act and is of considerable conservation concern. More study is needed! Unknown in the West and as yet unknown away from the island of Newfoundland or Anticosti Island, Quebec: it is unknown at this time whether it moves to nearby areas of mainland Canada or to offshore islands such as Magdalen Islands. A team of us are currently studying this call type, please upload any birds you might have recorded!
Cassia Crossbill (Benkman et al. 2009) — Large-billed (formerly Type 9)
Taxonomy: Loxia sinesciurus. Formerly known as “South Hills Crossbill” or “Type 9”. Was recently elevated as a species as Cassia Crossbill Loxia sinesciurus and is already a major conservation concern: http://www.hcn.org/articles/endangered-species-will-the-wests-newest-species-go-extinct
Known range: Restricted to South Hills and Albion Mountains of southern Idaho, in Twin Falls and Cassia Counties only [eBird map]
Movements: Apparently resident, but may rarely wander to mountains to northeast of South Hills (Benkman et al. 2009).
Preferred trees: Uses local variety of Lodgepole Pine that has evolved in absence of cone-predating pine squirrels.
Flight call: Very dry dip-dip-dip or dyip-dyip-dyip; very distinctive.
The Cassia Crossbill spectrogram starts with an initial upward component, therefore looking a bit like the Type 1 spectrogram. The downward modulation of the flight call is consistently given in a lower frequency domain (below 4.0 kHz) than Types 1 and 2. Like Type 1 it occasionally produces calls polyphonically (see Type 5 for discussion on polyphony) and also has secondary ending components. Overall, the Cassia Crossbill flight call sounds much lower and there’s a flat harsh quality to them. To our ear the dip-dip-dip call actually has an agitated quality to it. It sounds a bit like Type 11.
Status: West: This type was initially described as a full species–the South Hills Crossbill (Loxia sinesciurus)–by Benkman et al. in 2009 (Benkman et al. 2009). Starting in 1996, Benkman began studying this resident call type of Red Crossbills in the South Hills and Albion Mountains of southern Idaho. Cassia Crossbill is adapted to feed on Lodgepole Pine (var. latifolia) in an area that lacks tree squirrels (e.g., Red Squirrel Tamiasciurus hudsonicus), a primary cone predator, and the crossbills thus are the primary predators on pinecones in those mountains. In the absence of mammalian cone predators, the South Hills Crossbill has been tightly coevolving with this specific variety of Lodgepole Pine, driving the South Hills Crossbills to have larger bills to access seeds in better and better protected cones. This type is thought not to wander much at all, perhaps wandering rarely to adjacent mountain ranges to the north (Benkman et al. 2009). AOU did not accept the original proposal to split this form in 2009 on the basis of the lack specimens and genetic work. Since then, a genetic study has been done on the entire complex except Type 8 (Parchman et al. 2016) and specimens were deposited in the Museum of Vertebrates, University of Wyoming. Type 9 was recognized as a species in 2017 (Cassia Crossbill) based on it being genetically distinct from the remaining call types, having different flight call vocalizations, and the fact that it breeds mainly in April-July and starts molting in July before completing molt in September (Benkman et al. 2009, Parchman et al. 2016); the other call types often go through a partial molt in fall and breed late summer and early winter.
Type 10 – Sitka Spruce Crossbill (Irwin 2010) – Small-billed to medium-billed
Taxonomy: Best matches L. c. sitkensis, but specimens identified as this form probably include other types of similar morphology such as Types 1, 3 or 4. Nomenclature in need of resolution.
Known range: Primarily the coastal Pacific Northwest of northern California to central Oregon, but also appears to “irrupt” eastward occasionally into the Great Lakes, Northeastern U.S. and southeastern Canada. It’s hard to know how common it is in the east with the uncertainty of many birds in the east spectrographically looking and sounding like Type 7. Are these eastern birds all type 10 or are manly Type 7. See Type 7 for more. For now all of these Type 10/7 are being logged into eBird as Type 10 until further study clarifies the situation.
Movements: Irruptive, with a sizeable incursion to the Northeast in 1997-98. Has not been documented in the Rockies.
Preferred trees: Sitka Spruce in the Pacific Northwest; uses other spruces and pines in the East
Flight call: Very dry thin whit-whit; recalls Empidonax flycatcher whit note; very distinctive. Compare with Types 4 and 7.
The idea that there was a crossbill call type that gave a flight call similar to Type 4 but lacked a strong downward component had been known for several years, but Ken Irwin was the first to describe it formally and clarify its apparent ecological relationships (Irwin 2010). The flight call of Type 10 is perhaps one of the easiest call types to recognize. It’s a very thin, slightly weak whit–whit–whit. The whit-whit-whit sounds much like the whit call of an Empidonax flycatcher (e.g., Least, Dusky, Gray, Willow, or Buff-breasted Flycatcher). The spectrogram is dominated by an upward component. There are distinct differences between Type 4 and 10 spectrograms, with Type 4 containing a downward and upward component and Type 10’s usually just giving the upward component. Type 10 spectrograms do appear to be more variable than most of the other types (Irwin 2010), however, some have always been uneasy with the amount of variability included under Type 10. Some of these birds recorded and identified by Irwin could be Type 7 (see his examples 30, 32 and 33). Whereas Type 4 is a downsweep followed by an upsweep, and Type 7s an upsweep followed by a downsweep, classic type 10 is typically only an upsweep. Type 10 flight call can sound a bit like a White-winged Crossbill’s weet-weet-weet call, but is more pure toned and less nasal sounding.
Status: West: This type’s core zone of occurrence is in the Pacific Northwest, where it’s most closely associated with Sitka Spruce (Irwin 2010). Irwin has found large flocks of Type 10 regularly in the Sitka Spruce forests from coastal northern California to central Oregon (Irwin 2010), and there are records as far northwest as the Kenai Peninsula of Alaska. It should also be noted that Benkman (1993a) predicted a Sitka Spruce associating crossbill type many years ago. East: With many birds in the Northeast being a better match for type 7, it is unknown at this time exactly how common Type 10 is in the Northeast. In Groth’s (1993) initial description of crossbill types, eastern records of Type 10 were lumped with Type 4. It certainly does occur in the Northeast, and quite likely irrupts with Type 3 when they move east. In the great 1997-98 irruption the majority appeared to be Type 10 (identified at that time as Type 4), but some Type 3 were involved as well. In the Northeast, they seem to associate with various spruces, but will also readily snack on Eastern White Pine as well.
Type 11 – Central American Crossbill (Young and Spahr 2017) — Large-billed
[Important note: This taxon is not yet available in eBird for data entry, but will be added for August 2018. Please keep these recordings as “Red Crossbill” for now, and you’ll be able to update them once this taxon is added]
Taxonomy: Matches L. c. mesamericana
Known range: Guatemala, Honduras, Belize, Nicaragua, and El Salvador (recordings needed); [eBird map]
Preferred trees: Hard-coned pine species of Central America, most specifically Mexican Yellow Pine Pinus oocarpa (pers. comm. John van Dort)
Flight call: Type 11 birds give a flat, polyphonic flight call that can sound similar to the Cassia Crossbill or some lower-frequency variants of Type 5. It could be represented by drip-drip-drip, and sounds very different than the slightly ringing quality heard in the Type 6 birds with which it shares habitat.
Status: Type 11 seems fairly common in high-elevation conifers, particularly Pinus oocarpa, in Central America into southern Mexico. However, relatively little is known about its movements and limits of distribution. Thus far John van Dort has been instrumental in obtaining several recordings of this type in the field, but more study, and more recordings and measurements (preferably of the same birds) are still needed!
White-winged Crossbill (Gmelin 1789)
Taxonomy: Loxia leucoptera leucoptera. Unlike Red Crossbill, there’s no “call type” differentiation in the White-winged Crossbill in North America. Subspecies bifasciata is found across Eurasia, and, with further study, will likely be split to a full species given its notable differences in calls and ecological associations.
Known range: Breeding across the boreal forest of Canada with occasional nesting in the northeastern states, Cascades to central Oregon, central Rockies to northern New Mexico, and along coastal areas of western Canada; in irruption years will move south into Plains and Appalachians. [eBird map]
Movements: Highly irruptive every 5-15 years into the Northeast, Rockies and even the Plains. As with most other crossbills, has three main movements a year: one in May-June when it searches for newly developed cone crops; another in October after breeding and molting; and sometimes a third in December-January looking for the last remaining good cone crops.
Preferred trees: Tamarack and White and Black Spruce across the closed boreal forest of Canada; additionally uses Red Spruce in the northeastern states and southern Maritimes, Engelmann Spruce in Rockies, and rarely Sitka Spruce along coastal Alaska. Will use a variety of conifers during irruptions south of normal breeding range too.
Flight call: The main flight call given by the White-winged Crossbill is a chattering very redpoll-like chyet–chyet or chet–chet call, usually doubled or tripled, and repeated once every second or two. This White-winged Crossbill flight call is a louder, twangier and more musical version of a redpoll flight note. Audiospectrographic analysis of the White-winged Crossbill is of somewhat less importance as long as you get the basic gist of their calls down. With some practice, this standard flight call of the White-winged Crossbill can be easily distinguished from any Red Crossbill vocal type or other finch species. White-winged Crossbills also give a second call that can be easily confused with the call of various Red Crossbill vocal types; however, it’s a sharper, quicker and thinner (veet–veet–veet) than any of the Red Crossbill vocal types. These veet–veet calls are more commonly given by perched birds, which may help separate from Red Crossbill flight calls. Please note White-winged Crossbills also give a rising whee call that is similar to some redpoll, siskin, or goldfinch flight calls. This rising call is never given by Red Crossbills.
Status: Common across the closed boreal forest of Canada; uncommon in the northeastern states and southern Maritimes; local in the Cascades and Rockies; and rare in the Plains and south of New York in the eastern United States.
Hispaniolan Crossbill (Banks et al. 2003)
Taxonomy: Loxia megaplaga. Recently considered part of the White-winged Crossbill species complex, Hispaniolan Crossbill was designated as a separate species in 2003 (Banks et al. 2003). The species has been isolated from White-winged Crossbills since the Pleistocene (Woods et al. 1992). The Hispaniolan Crossbill differs significantly from the White-winged Crossbill in vocalizations and in bill morphology. The bill of the Hispaniola Crossbill is 25% larger than that of the White-winged Crossbill, and is closer in size to the Red Crossbill (Loxia curvirostra; Benkman 1994, Parchman et al. 2007).
Known range: Endemic to the island of Hispaniola. [eBird map]
Movements: Non-migratory and resident to the island of Hispaniola.
Preferred trees: Primarily eats seeds of Hispaniolan pine (Pinus occidentalis)
Flight call: A dry or wooden chit–chit–chit
Status: Endangered and rare in high-elevation pine forests of Hispaniola. Available habitat is decreasing as a result of fire, logging, and conversion of land to agriculture.
Understanding how these differences relate to traditional taxonomy is fraught with complexity and is an area in need of additional research. One key component of any of this research is plenty of recordings of flight call types (optimally with measurements from the same recorded individuals). Part of the complexity is in nomenclature, since it is unclear how present Types relate to named subspecies from the past. This is due in part to the fact that some of the named subspecies do not seem to be identifiable and multiple names may apply to a given population and/or Type (i.e., some named subspecies are not valid). Additional complexity arises from the fact that we don’t understand the extent to which these different types are reproductively isolated and whether or not they are behaving as species according to the Biological Species Concept. Cassia Crossbill was split because it was found to be reproductively isolated — hybridization was found to be less than 1% (Benkman et al. 2009), whereas between Scottish, Parrot and Common Crossbill it was 5% (Summers et al. 2007). It could well be that some types represent distinct species, or it could be that they are better treated as distinct forms that have not yet evolved to represent distinct species. Thus, the study of various flight call types of Red Crossbills may inform us of our broader definition of what it means to be a separate species.
The 2017 eBird/Clements taxonomy (v2017) includes not only the nomenclature and taxonomy of species, but also of subspecies. Subspecies cannot be reported in eBird unless it is included as an identifiable group, but Red Crossbills can be reported to Type in eBird.
Every crossbill recording adds an important piece to the puzzle, especially when accompanied by notes on behavior and ecology, including tree species used for foraging and nesting. The conservation of crossbill call types will depend in large measure on our understanding of their complex distributions and ecological associations, and birders can make critical contributions to their conservation by recording crossbill calls and by reporting their findings.
Since an understanding of conifer species is essential to understanding crossbills, the above article discusses conifer species at some length. Below is a list of the scientific names (with Wikipedia links) for the tree species mentioned in the article.
Pines — Genus Pinus
Pines can be broken down further into soft-coned pines (the two White Pine species listed below) and hard-coned pines (all the others listed below). At some points in the above article we refer to hard-coned or soft-coned species.
Douglas-firs — Genus Pseudotsuga
Spruces — Genus Picea
Hemlocks — Genus Tsuga
Larches — Genus Larix
Below are links to several cuts with multiple types in it. To hear a great comparison between Type 1 and Type 3 calls, try this link. In the first 10 seconds, the recording has a Type 1, followed by Type 3, and then Type 1 again, making for a particularly good chance to compare the calls side-by-side. To hear the differences between some of the toughest ones to differentiate, listen to this cut with Type 1, unkinked Type 2 and Kinked Type 2 (examples of “kinked” Type 2 are at 4s, “unkinked” Type 2 at 11s, and Type 1 at 14s.)
To hear a cut with similarly sounding Type 4 and 6 (and an “unkinked” Type 2 at 14s mark), listen here. The cut starts out with Type 4 and then Type 6 start to come in at the 3s and 5s marks, and are mixed in throughout the rest of the cut. The Type 6 is slightly higher in frequency, but has a narrower frequency range.
Lance Benner’s wonderful cut from Arizona has great examples of Types 2, 4 and 5, which to a trained crossbill ear, are fairly easily to differentiate in the field compared to the types in the above clips. Click here. At the 33s mark the flock takes flight and the majority of the birds are Type 5s initially, but at the 36s, 42s and 44s marks Type 2 can be seen and heard mixed in. At the 46s and 48s marks the first “V-shaped” Type 4 can be seen and heard, and then after the 1:06 mark until 1:19 it’s entirely Type 4.
To hear a long, and very interesting crossbill recording that includes a variety of songs and different crossbill flight calls, click here. In addition to the full gamut of crossbill sounds that one individual might produce (flight calls, toops, “social” songs, chitters, etc.), this amazing cut includes four different call types: Type 1, Type 2, Type 3, and Type 10. See if you can sort them out!
We would like to thank the Macaulay Library at The Cornell Lab of Ornithology, The Borror Laboratory of Bioacoustics, and The Museum of Vertebrate Zoology at Berkeley (aka Jeff Groth’s collection) for the use of recordings. A special thanks goes out to Craig Benkman, Jeff Groth, and Tom Hahn for getting us all started down the crossbill road – without their research, we would not be where we are today! I’d also like to thank Rodd Kelsey, Cody Porter, Lance Benner, Nathan Pieplow, Ken Irwin, Tayler Brooks, Andrew Spencer, Bob Dunlap, John van Dort,Anant Deshwal, Jamie Cornelius, Susan Kielb, Nick Anich, Ryan Brady, Junior Tremblay, Doug Hynes, Christophe Buidin, Yann Rochepault, Olivier Barden, Michel Robert, Joseph Neal, Kimberly Smith, Mike Nelson,Pooja Panwar, Doug Robinson, Doug Hitchcox, Louis Bevier, Magnus Robb, Julien Rochefort, Patrick Franke, Tom Reed, Steve Kolbe, Dave Slager, Marilyn Westphal,Walter Szeliga, Ken Blankenship, Joan Collins, and all the other growing number of Loxiaphiles for continued input to an evolving and spirited discussion about crossbills. A special thanks to Michael O’Brien for edits and comments to this document! Lastly, we’d also like to thank the eBird team for advice, editing, and contributions to this piece.
The below recordings are credited to The Macaulay Library at The Cornell Lab of Ornithology:
Type 1_flight_call_ML137497 – Gregory F. Budney
Type 1_flight_call_ML22786381 – Matthew A. Young
Type 2_flight_call_kinked_ML44960 – Geoffrey A. Keller
Type 2_flight_call_unkinked_ML161299 – Matthew A. Young
Type 3_flight-call_ML68287331 – Andrew Spencer
Type 3_flight_call_ML71525571– Matthew A. Young (formerly ML136592)
Type 4_flight_call_ML68287801 – Andrew Spencer
Type 4_flight_call_ML58167 – William W. H. Gunn
Type 5_flight_call_ML138539 – Gregory F. Budney
Type 5_flight_call_ML516122 – Mark B. Robbins
Type 6_flight_call_ML42530661 – Timothy Spahr
Type 7_flight_call_ML12985 – Robert C. Stein (provisional ID)
Type 7_flight_call_ML64320431 – Tom Auer (provisional ID)
Type 8_flight_call_ML134103 – Martha J. Fischer
Type 10_flight_call_ML22786541 – Matthew A. Young (formerly ML136593)
Type 10_flight_call_ML68288161 – Andrew Spencer
Type 11_flight_call_ML50907531 – John van Dort
Cassia Crossbill_flight_call_ML64374391 – Andrew Spencer
Cassia Crossbill_flight_call_ML65163941 – Jay McGowan
White-winged Crossbill_ML103203 – Matthew D. Medler
Hispaniolan_Crossbill_12989 – George B. Reynard
The below recordings are credited to The Museum of Vertebrate Zoology at Berkeley:
Type 6_flight_call_aF743– Jeff Groth
Type 7_flight_call_aF497 – Jeff Groth
Type 7_flight_call_jM498 – Jeff Groth
Benkman, C. W. 1993a. Adaptation to single resources and the evolution of crossbill (Loxia) diversity. Ecological Monographs 63: 305-325.
—-. 1993b. The evolution, ecology, and decline of the Red Crossbill of Newfoundland. American Birds 47:225-229.
—-. 2007. Red Crossbill types in Colorado: their ecology, evolution and distribution. Colorado Birds 41:153-163.
Benkman, C. W., J. W. Smith, P. C. Keenan, T. L. Parchman, and L. Santisteban. 2009. A new species of Red Crossbill (Fringillidae: Loxia) from Idaho. Condor 111: 169-176.
Griscom, L. 1937. A monographic study of the Red Crossbill. Proceedings of the Boston Society of Natural History 41:77-210.
Groth, J. G. 1993. Evolutionary differentiation in morphology, vocalizations and allozymes among nomadic sibling species in the North American red crossbill (Loxia curvirostra) complex. Univ. California Publications in Zoology 127: 1-143.
Hynes, D.P. and E. H. Miller. 2014. Vocal distinctiveness of the Red Crossbill (Loxia curvirostra) on the island of Newfoundland, Canada. Auk 131(3):421-433. [link]
Irwin, K. 2010. A new and cryptic Call Type of the Red Crossbill. Western Birds 41: 10-25.
Kelsey, T. R. 2008. Biogeography, foraging ecology and population dynamics of red crossbills in North America. Doctoral Dissertation, University California, Davis, December 2008.
Parchman T. L., C. W. Benkman, and S. C. Britch. 2006. Patterns of genetic variation in the adaptive radiation of New World crossbills. Molecular Ecology 15: 1873-1887.
Parchman, T. L., C. A. Buerkle, V. Soria-Carrasco, and C. W. Benkman. 2016. Genome divergence and diversification within a geographic mosaic of coevolution. Molecular Ecology 25:5705-5718.
Pieplow, N. 2007. Colorado Crossbill Types: 2, 4 and 5. Colorado Birds: 41: 202-206.
Sewall, K. B. 2009. Limited adult vocal learning maintains call dialects but permits pair-distinctive calls in Red Crossbills. Animal Behavior 77: 1303-1311.
—-. 2010. Early learning of discrete call variants in red crossbills: implications for reliable signaling. Behavioral Ecology and Sociobiology 65: 157-166.
Spencer, A. 2009. Specimens of rare Red Crossbill types from Colorado. Colorado Birds 43(3): 187-191.
Summers, R. W., R. J. G. Dawson, and R. E. Phillips. 2007. Assortative mating and pat- terns of inheritance indicate that the three crossbill taxa in Scotland are species. Journal of Avian Biology 38: 153-162.
Szeliga, W., L. Benner, J. Garrett, and K. Ellsworth. 2014. Call types of the red crossbill in the San Gabriel, San Bernardino, and San Jacinto Mountains, southern California. Western Birds 45: 213-223.
Young M. A. 2010. Type 5 Red Crossbill (Loxia curvirostra) in New York: first confirmation east of the Rocky Mountains. North American Birds 64: 343-346.
—-. 2011. Red Crossbill (Loxia curvirostra) Call-types of New York: their taxonomy, flight call vocalizations and ecology. Kingbird 61: 106-123.
Young, M. A., K. Blankenship, M. Westphal, and S. Holzman. 2011. Status and distribution of Type 1 Red Crossbill (Loxia curvirostra): an Appalachian Call Type? North American Birds 65: 554-561.
Young, M. A., D. A. Fifield and W. A. Montevecchi. 2012. New evidence in support of a distinctive Red Crossbill (Loxia curvirostra) Type in Newfoundland. North American Birds. 66: 29-33.
*Matthew A. Young, Cornell Lab of Ornithology, 159 Sapsucker Woods Road, Ithaca, NY 14850.