Identifying the seven species of Pacific Northwest swallows in spring is a fairly simple matter, given reasonable views. But, the same cannot be said for those same species in the fall, resulting in a fair number of erroneous reports and requests from the eBird editors for additional documentation. What makes fall swallow identification so much more problematic than spring? There are two major factors: confusing plumages and misconceptions about migration timing in the Pacific Northwest.
Fall swallow flocks contain many juveniles and worn adults, both of which pose field identification challenges that are not an issue in spring or early summer. They are often seen as large, distant swirling migratory flocks, also not conducive to easy identification. All of the Northwest swallows have one complete molt per year, their prebasic (post-breeding) molt, and have little or no prealternate (pre-breeding) molt, so by late summer adults are quite worn, making many of the usual field marks more difficult to discern. Juvenile plumages are more similar among the species than the adult. Some common fall identification problems include:
- Tree vs. Violet-green – The large white patches on the sides of the rump and over the eye that make spring and summer Violet-green Swallows so conspicuous can be much less distinct, or even absent in both worn adults and juveniles, particularly when viewed from a distance. Violet-green Swallow prebasic molt occurs primarily on the summer grounds, so birds in August and September in the Northwest are in molt; and so, most lingering birds found in October and later are in fresh basic plumage and are easier to identify. In contrast, Tree Swallow prebasic molt for both juveniles and adults commences on the summer grounds and completes on the winter grounds, so fresh fall birds are unlikely.
- Tree vs. Bank – Many juvenile Tree Swallows show an incomplete breast band, and a few show almost complete bands, creating confusion with Bank Swallow. Observers searching for Bank Swallows in the fall should look for their smaller size, different back pattern and color, and different face pattern, and not just rely on presence of a breast ban. One interesting difference is that Bank Swallows molt all of their flight feathers on the wintering grounds, so a brown-backed swallow showing wing molt in the Northwest is probably not a Bank.
- Barn vs. Cliff – Juvenile Barn Swallows are often strikingly different than adults, with much shorter tails and much lighter underparts, and as a result have been mistaken for Cliff Swallows, or even the white-bellied swallows. Their tail fork projection is only 1-2cm, while adult Barns have tail projections of 2.5 to >5cm (Pyle 1997). Since both Cliff and Barn molt primarily on the winter grounds, fall adults in the Northwest are often quite worn and Barn can have broken tail streamers.
Many Pacific Northwest observers are not aware that fall migration timing is very different in our region than in the eastern part of the continent. For instance, along the Atlantic Coast, Tree Swallows are typically the last species to depart, with birds regular to mid-November and lingering through December as far north as Massachusetts
In contrast, the Pacific Northwest Tree Swallows are one of the first species to depart, with only a handful of lingering birds after late September
This table uses the eBird Abundance indicator to estimate departure dates, based on when their Abundance reaches 0, see for example this Abundance chart for five widespread Washington swallows.
|Washington Departure||Oregon Departure|
|N. Rough-winged Swallow||Sep 1||Sep 1|
|Purple Martin||Sep 15||Sep 15|
|Tree Swallow||Sep 1||Sep 22|
|Violet-green Swallow||Oct 15||Oct 22|
|Bank Swallow||Sep 22||Sep 15|
|Barn Swallow||Oct 22||Oct 22|
|Cliff Swallow||Sep 1||Sep 1|
 This date may reflect a significant amount of misreporting. The line graphs for Tree Swallow in Oregon show large fluctuations in birds per hour and average count from Sep 1 to Oct 15, potentially indicating wide discrepancies between observers. Similar fluctuations are not evident earlier in the year. A comparison of Violet-green and Tree Swallow line graphs in Oregon shows that many fewer observers are reporting Tree Swallows in September than are reporting Violet-green Swallows, and that the trend in the birds per hour graph for Violet-greens is consistent with the trend in the abundance or frequency graphs, so the increased variability in birds per hours in fall for Violet-green is a reflection of their patchy distribution during fall migration.
Why do west coast Tree Swallows show a very different fall migration pattern than eastern birds? One clue may lie in the Central Valley of California. Massive Tree Swallow roosts have been recently documented in the valley. The abundance chart for Tree Swallow in four counties (Fresno, Merced, San Joaquin, Yolo) demonstrates this rather dramatically. The size of the roosts are so large that the counts from the rest of the year appear almost nonexistent by comparison. Cousens, Rowoth and Airola (2010) describe the discovery of several large Tree Swallow roosting sites in the central valley. Tree Swallows appear to leave the Northwest early to take advantage of suitable conditions in the Central Valley; east coast Tree Swallows appear not to have similarly attractive stopover points on their southbound migration.
What does this mean for eBirders?
Knowledge of both identification challenges and migration timing will help eBirders improve the quality of their checklists. For example, a distant, large flock of white-bellied swallows in late September in the Puget Trough can be safely identified as Violet-green, even if other field marks are not visible. September checklists reporting Cliff, Northern Rough-winged or Tree should include some details about the field marks that were noted.
Interested in trying to find swallow roosts, and documenting them in eBird? The note by Cousens and others on verifying Tree Swallow roosts in the central valley includes considerable information on how they were initially detected on Doppler radar, prior to being verified in the field. Oregon observers have done a great job of documenting the similar Barn Swallow roosts in the upper Willamette Valley, the challenge remains for observers elsewhere in the region to uncover other roosts.
Read more eBird Northwest species articles here.
Cousens, B., J. Rowoth and D.A. Airola. 2010. Large Tree Swallow roost verified in California’s Central Valley. CVBC Bulletin 13 (1): 1-17.
Pyle, P. 1997. Identification Guide to North American Birds Part I Columbidae to Ploceidae. Slate Creek Press, Bolinas, CA. 732pp.
Article written by: Bill Tweit