Ash-throated Flycatchers fleeing the drought?

The movements of rare birds have long been the subject of theories and postulations. How did they get here? Is there a pattern? Can their appearance be predicted? For Ash-throated Flycatchers in the East, a pattern has been understood for about three decades. Though rare, they predictably appear on the East Coast (Florida to Atlantic Provinces) from early November to mid-December. In fact, during this season one is more likely to encounter an Ash-throated Flycatcher on the East Coast than a Great Crested Flycatcher (which typically depart by the first week of October). Recently, this has coincided with a growing phenomenon of Cave Swallows during the same time frame. But earlier records (of Cave Swallows and Ash-throateds) are quite rare.

Half-way through the fall of 2011 though, something seems different. Already in the East there have been more Ash-throated Flycatchers than ever before in October. So far this year, the following well-documented records are shown in eBird:

27 Sep 2011, Plum Island, MA
7-9 Oct 2011, two birds at Plum Island, MA (one possibly a continuing bird)
11-12 Oct 2011, Boston, MA
15 Oct 2011, Grand Manan I., NB
16 Oct 2011, Cuttyhunk I., MA
17-18 Oct 2011, Cape May, NJ
23 Oct 2011, Monhegan I., ME
28 Oct-1 Nov 2011+, Salisbury State Reservation, Essex Co., MA

On the East Coast, eBird shows just one prior record from September (9 Sep 2000 at Jamaica Bay WR, Queens, NY). The six prior records from October in eBird are all from 25 October or later. The new eBird maps provide an opportunity to examine records month by month:

Ash-throated Flycatchers in September
Ash-throated Flycatchers in October
Ash-throated Flycatchers in November
Ash-throated Flycatchers in December

Of course, we are not yet at the point where all historical bird records are entered in eBird, and this day might never come, but perhaps all rarity records will make it into eBird eventually. In fact, some states like Vermont, Massachusetts, and Colorado already have their Bird Records Committee (BRC) databases entered in eBird. This makes researching patterns on rarities much more interesting and rewarding (please get in touch at ebird@cornell.edu for tips on how to get your state’s BRC data in!).

So, are the nine 2011 records from late September to October really that unusual? To check on this, we can dive into older literature on bird occurrence in the East. Through 1993 in Massachusetts (Veit and Petersen 1993) there were 8 fall records (and one spring record), all after 1 November. In New York, Levine (1998) lists 8 fall records (and two spring), all occurring 2 Nov or later except for one at Riis Park, Queens, on 24 Oct 1992. Of 11 accepted records in New Jersey through 1999 (Walsh et al. 1999), all occurred 11 Nov-10 Jan, except for one record 24-31 Oct 1987 at Sandy Hook. Of 14 accepted records in Maryland (MD/DCRC data), all have occurred 11 Nov-19 Dec, except for one exceptional bird 29 Sep 2001 on Assateague Island. In Virginia, the 16 state reports through 2007 (Rottenborn and Brinkley 2007) are all from 6 Nov-6 Mar. This is just a sampling, but additional records from the Carolinas, Delaware, Pennsylvania, Connecticut, Rhode Island, New Hampshire, Maine, and Atlantic Canada follow a similar pattern: most records from early November to early January, with very few earlier records.

So if we have established that October 2011 was indeed unusual, what could account for so many Ash-throated Flycatchers? First, it is important to acknowledge a few generalities about birding today and how that relates to rarity discovery and documentation. Since many more birders are birding today and are much better connected (via eBird, regional listserves, and instant cell phone contact), the proportion of rarities that get discovered seems to be higher overall. Second, with the advent of digital photography, the proportion that are well-documented and end up in the permanent record has increased even more. And third, our collective awareness to always check late season Myiarchus and Empidonax flycatchers, hummingbirds, warblers, and other out-of-season finds for rarer western species is much more advanced than it was two decades ago. Now, many birders instantly respond to any report of these species to check for western rarities.

But these changes in birding coverage and awareness have been gradual, and 2011 still stands out from all other years in the past decade in terms of the number of October Ash-throateds in the East.

Interestingly, the Ash-throateds this October have invariably been in juvenal plumage, with all rufous tails and very worn wings. While immature flycatchers are almost always the ones to turn up out of range, their appearance in this plumage state suggests that these birds were on the move before molting. This is another way that these birds are unusual in comparison to past records. Most November/December Ash-throateds in the East have been young birds, but have already molted adult tail feathers and body feathers (often with juvenal feathers still in the wings). These fresh-plumaged immature birds arrive primarily in conjunction with southwesterly wind flow in November–the same winds that have produced eastern Cave Swallows recently. While theoretical, that connection does seem to make sense since both birds occur commonly in a similar area of Texas. The long cold fronts that signal these Cave Swallow movements provide southwesterly winds (in advance of the front) that literally connect central Texas with the Great Lakes and/or Northeast. So it seems reasonable to think that the swallows and flycatchers are on the move in Texas/Northeast Mexico and can get displaced when these extensive cold fronts occur. But the earlier movement of the 2011 birds–and their appearance before molting–may mean that something different is occurring.

This year has been unusual for the record-setting drought in Texas and the southern Great Plains. Since Texas and northeast Mexico is a center of Ash-throated Flycatcher breeding range, it may be that fledged birds are somehow fleeing the drought and that a few of these birds are turning up more widely. Their ultimate appearance on the East Coast is likely a result of the strong southwesterly winds that have dominated much of October, but the larger question is why these birds were on the move at all. (Vagrant birds do not just get “swept up” in winds, but birds on the move do seem to drift or even engage in “wrong way” flights when such winds occur.)

Drought data provided by the US Drought Monitor. Note the region of extreme drought in the Southwest, and how that corresponds with the typical distribution of Ash-throated Flycatcher in Aug-Sep (below).

The phenomenon of molt migration is common in western birds (less so for Eastern species), and species following this pattern often move to new areas (sometimes moving from the Northwestern US to western Mexico, or similar distances) after breeding or fledging to undergo their molts in areas with wetter conditions and more plentiful foraging resources. These migrations often happen before true “fall migration” to wintering grounds, so are effectively the first of two post-breeding migrations, as birds move between breeding and wintering grounds.

Do Ash-throateds from the western US and northern Mexico have a molt migration? If so, it might make sense that Ash-throated Flycatchers that move into drought-stricken areas would find these regions unsuitable for molting (molting is typically done in areas with rich food resources) and might disperse more broadly. Perhaps the unique combination of Ash-throateds moving out of molting grounds earlier, before molting, and the dominant southwesterlies have combined to produce these records? If so, what other birds might be showing similar patterns? Cassin’s Sparrows and Dickcissels obviously fled the drought this spring and summer, but what else is going on this fall? We’d love to hear from eBirders on these questions.

Using eBird observations in conjunction with field impressions from birders on the ground can often help refine theories like this regarding bird vagrancy and its causes. In this case, it seems likely that the Southwest drought is connected to this pulse of early Ash-throateds. It would be very interesting to hear from Texas, New Mexico, and north Mexico birders about the occurrence of Ash-throateds there this year (especially late summer/fall) and whether it has seemed unusual in any way.

Your continued reports to eBird, of all birds (rare and common alike), will help us to investigate questions like this in greater detail.

References:

Levine, E. (ed.). 1998. Bull’s Birds of New York.

Rottenborn, S. and E. S. Brinkley. 2007. Virginia’s Birdlife: An Annotated Checklist.

Walsh, J., V. Elia, R. Kane, and T. Halliwell. 1999. Birds of New Jersey.

Veit, R. R. and W. R. Petersen. 1993. Birds of Massachusetts.