Flycatchers are a large group of insect-eating birds that reach their highest diversity in the tropics. For North American birders, and indeed for birders everywhere, this group often causes identification headaches, especially distinguishing the very similar species in the genus Empidonax. Many of these confusing flycatchers share drab plumages; often clad in various shades of grays, greens, and olive. While the identification of these birds is covered in good detail in most North America field guides, many of these same guides neglect to highlight the importance of migration timing as an aid in the identification process. By knowing what species to expect in your region at a given time of year, you can quickly limit your realistic choices, and focus on distinguishing just a few of the most probable species.
Whenever birders are confronted with difficult identification challenges, it’s helpful to know what birds are expected in a region. The range maps in field guides help with this by showing where birds are expected to occur at different times of the year; typically breeding, migration, and winter. But bird occurrence is more complicated than that, and patterns differ from region to region. If you’re a Texas birder, you can expect to find migrants arriving and departing on a different schedule than you would in New York. Makes sense right? It takes birds a certain amount of time to get from Point A to Point B, and it’s not rocket science to infer that birds moving north in spring arrive in Texas before they arrive in New York!
With these patterns in mind, you can use the eBird ‘Line Graphs’ to explore migration timing of similar species in your region. By comparing the timing of migration of these birds, you can begin to limit your choices depending on when and where you are.
Below are two graphs that show how migration timing differs for three often confused flycatcher species in Texas and New York: Eastern Phoebe (blue), Eastern Wood-Pewee (red), and Willow Flycatcher (green). Click on the image to see a larger version.
Migration timing of three flycatchers in TX.
Eastern Phoebe winters in large numbers in Texas, and as you can see above, it begins to decrease in frequency (= spring migration) by March. The other two species, however, do not winter in Texas, and these come through much later in spring. The Eastern Wood-Pewee is next to arrive, with spring migration through Texas peaking in late April and early May. Willow Flycatcher is even later, moving through in mid-May. The take home message here from an identification standpoint is this: if you’re seeing a confusing flycatcher in March that you think is one of these three species, it’s almost certainly an Eastern Phoebe. Even in late April your choices are limited to Eastern Phoebe and Eastern Wood-Pewee, with Willow Flycatcher a real long-shot at that time of year. By mid-May you can expect all three in TX, though most of the wintering and migrant Eastern Phoebes are long gone.
Migration timing of three flycatchers in NY.
None of these three species winter regularly in New York, and on the graph above we can see very clear-cut spring arrivals for all three. Perhaps not surprisingly, the overall pattern is similar, with Eastern Phoebe arriving earliest, Eastern Wood-Pewee next, and Willow Flycatcher last. The entire migration is pushed back several weeks from what you see in Texas. Migrant Phoebes really peak in early April, when neither of the other two is a realistic possibility in New York. A month later in early May, however, you need to be more careful, as all three could be in the state. Even then there are differences, with Eastern Wood-Pewee peaking in mid-May and Willow Flycatcher in late May.
This group of small, drab, wing-barred flycatchers is notoriously difficult to identify. Subtle differences in appearance and structure are helpful clues, but voice is the best way to distinguish them. On migration, however, birds are often not calling, and observers are frequently perplexed by them, leaving the identification as a very reasonable ‘Empidonax sp.’. But there are helpful clues to be had in examining the differing migration timing among Empids. The graph below shows the migration intervals for four Empids in New Jersey: Least Flycatcher (blue), Acadian Flycatcher (red), Willow Flycatcher (green), and Yellow-bellied Flycatcher (purple).
Migration timing of four Empidonax flycatchers in NJ.
In the graph above we can see two important things: the spring migration peaks of all four species; and their overall frequency of occurrence. A visual examination of the spring migration peaks helps us limit our realistic choices, whereas an understanding of the frequency helps us know how ‘common’ a bird is in the region.
The first two species to arrive in late April are Least and Acadian Flycatchers. The interesting thing here from an identification standpoint is that when you consider the date and location, only two species are probable. Those two species happen to be quite different. Least Flycatcher is small and compact, with short wings and a small bill. Acadian Flycatcher is larger and longer-winged, with a larger bill that is mostly pale underneath. So even if you encounter a silent Empid in New Jersey in late April, your realistic choices are already limited to Least and Acadian, and distinguishing those two species usually is possible given reasonable views.
By mid-May things become more complicated, as Willow and then Yellow-bellied flycatchers arrive. Looking at this graph we can still gain more information: Yellow-bellied occurs in relatively lower frequency, and appears to arrive at least a week later than Willow on average. So it’s really not until late May that Yellow-bellied is moving through the state in numbers.
So much of successful bird identification hinges on knowing what birds to expect in a given region. Adding an understanding of migration timing to your arsenal will greatly improve your ability to take your identification skills a step further. eBird is a great resource for exploring these patterns at different scales. You can use the ‘Bar Charts’ to select your region of interest, and then click on the species names to explore the line graphs more closely. As more data come into eBird, these tools become all that much better. Thanks for participating, and please take a minute to try to convince other birders to contribute their observations to eBird too!