David M. Larson
In the fall, Blackpoll Warblers (Setophaga striata) are celebrated for the exercise in identification and for the appreciation of their long and difficult impending over-water migration. However, in the spring, when local birders find these warblers, a shading of existential dread often colors the experience. "Oh no, migration is almost over!" Maybe that is why relatively little has been published about spring Blackpoll migration.
Because Blackpolls breed across the North American continent in the boreal forests of the United States and Canada and migrate from their South American wintering grounds past a broad array of bird banding stations, they would seem to be a good model for studies on the drivers, modulators, and phenology of migration. They are also a rapidly declining species of concern, meaning that study could illuminate issues of conservation action or further research. Elucidating parameters of spring migration in Blackpolls was the goal of the research reported by Covino et al. 2020. The authors assembled spring records, collected between 1960 and 2017, from 28 birdbanding sites, stretching from Florida and Mississippi up the East Coast, across the upper Midwest, to western Canada and Alaska. Data were retrieved from the United States Geological Survey (USGS) Bird Banding Laboratory data base. The researchers also reached out to individual banding stations for additional data, such as wing chord and mass, not usually recorded in the USGS data base.
The hypotheses to be tested were (1) that the number of birds captured in western locations would peak later than those in eastern banding stations; (2) that western populations would have longer wing chords correlating with their longer migration distances; (3) that the energetic condition (mass compared to wing length) of the birds captured in the west and the east would differ; (4) that male Blackpolls would arrive earlier than females; and (5) that the migration phenology of Blackpolls would not show shifts over time considering the late timing of their migration. To test these hypotheses, the researchers examined how migration dates, wing length, and energetic condition varied with sex, location, and estimates of distance from overwintering grounds. Eastern and western banding station designations were separated by the 85o west longitude line, which corresponds to known population migration tracks. In the United States, this longitude line runs from the Florida panhandle through central Michigan.
The researchers statistically analyzed the data from nearly 16,000 new spring captures, although numbers were reduced by a few thousand for analyses of mass and wing length because these data were not available from all stations. The progression of migration varied with route, distance from wintering grounds, and sex, with the midpoint of migration for western birds almost 15 days earlier than that for eastern birds, and male midpoints about 4 days earlier than those for females.
Data on wing length showed two-way interactions, so eastern and western birds were analyzed separately. Males in both populations had longer wings than females. The magnitude of the sex difference was greater in western birds at distant sites than at shorter migration distances, while for the eastern birds, the differences were greater earlier in the season rather than later. In both eastern and western populations, birds with longer wings arrived earlier. The energetic condition of the birds showed more complex statistical interactions, so eastern and western birds were again analyzed separately. In both groups, longer-winged birds and males had more mass. The effect of migration distance on body mass was nonlinear for eastern birds, depending on sex and the date. Eastern and western birds captured in the south, near the Gulf of Mexico, were underweight (normalized by wing length), and birds in the Midwest were above expected mass. On the East Coast, mass was highest in the middle of the range and about as expected near the breeding grounds, while in the west, peak weights were in the mid-latitudes and were lower in the north.
Overall, 58 years of data from spring banding records indicate that Blackpoll Warblers have migrated earlier in the year by about a half day per decade whether comparing early, median, or late migrants. While unexpected, this result is in concordance with data from other Neotropical migrants. The earlier arrivals of western populations with similar migration distances suggest that both sexes of western birds begin migration earlier. The energetic condition of the birds on capture was similar for eastern and western populations, despite the differences in migration distances, mass being lowest in the south, highest in mid-track, and near optimal near the breeding grounds. The western birds do increase the speed of migration toward the end of the track, suggesting another difference in migration strategy. Eastern birds may be making a more deliberate migration while the western birds may make a big jump and spend more time fattening up in the Midwest.
Males migrate earlier in spring than females, but females apparently speed up for the last part of their migration and arrive in worse condition than the males. Doubtless the females make up any deficiencies upon arrival because they need to be in robust condition to produce a successful clutch of eggs. The differences in migration strategies might prevent intersex competition during migration and lead to more optimal success on the breeding grounds.
This type of study, using data from numerous banding stations, demonstrates the value of a broad view of information impossible with a more restricted data set. Further, the combination of springtime migration data with published information from the same research group on the fall migration patterns of Blackpoll Warblers (Morris et al., 2015) provides a more complete view of the annual life cycle of these birds, their evolved migration strategies, and possible conservation strategies for this rapidly declining Neotropical migrant.
References
- Covino, K. M., S. R. Morris, M. Shieldcastle, and P. D. Taylor. 2020. Spring Migration of Blackpoll Warblers across North America. Avian Conservation and Ecology 15 (1), Article 17. doi.org/10.5751/ACE-01577-150117.
- Morris, S. R., K. M. Covino, J. D. Jacobs, and P. D. Taylor. 2015. Fall migratory patterns of the Blackpoll Warbler at a continental scale. Auk 133 (1):41-51. doi.org/10.1642/AUK-15-133.1
David M. Larson, PhD, is the Science and Education Coordinator at Mass Audubon's Joppa Flats Education Center in Newburyport, the Director of Mass Audubon's Birder's Certificate Program and the Certificate Program in Bird Ecology (a course for naturalist guides in Belize), a domestic and international tour leader, past President of the Nuttall Ornithological Club, and a member of the editorial staff of Bird Observer.