This week has been surprisingly busy for the Lesser Slave Lake Bird Observatory’s spring migration monitoring program with 318 birds banded between June 2 and 8. Although we are capturing expected numbers of late season migrants, such as Canada Warblers, Alder Flycatchers, and even a rare Connecticut Warbler, our nets have also seen unusually high captures of Swainson’s Thrushes and American Redstarts which normally do most of their migration earlier in the season.
Though the timing is odd, this is a welcomed sight. Until this week, a strong majority of captured Swainson’s Thrushes were birds hatched earlier than 2021 with captures of birds hatched just last year few and far between. This was concerning since we recorded high numbers of young Swainson’s Thrush going south for the winter last fall, so why were none of them returning?
Observations of few young birds returning to their summer breeding grounds can be alarming and indicate that there is something causing excess mortality on their migratory pathways or on the wintering grounds. Since old birds have already migrated multiple times, they can better adapt to a modestly increased risk during migration. Young birds, however, are still trying to figure out how to migrate and any small mistake can turn deadly. With many opportunities to err, most young birds do not survive their first year and have little ability to adapt to increased risk. Furthermore, when habitat is destroyed on the wintering grounds, older birds are better able to compete for the limited resources and are more likely to survive.
This ability to track the age of birds moving through the area during spring and fall migration is a major reason why we capture birds. We often cannot obtain this vital data without having them in-the-hand. Additionally, Swainson’s Thrushes tend to migrate through the thick understory where it is almost impossible for us to detect them, unlike many other species that we can identify as they fly overhead.
Luckily, our young Swainson’s Thrush have finally shown up. Normally the adults arrive just a few days before last year’s young, but the gap before the young arrived this season in strong numbers was over two weeks. Why are the young so late when their parents arrived exactly when we expected them to? Honestly, we do not know and have not been able to brainstorm many strong hypotheses. However, we have begun to wonder if the long-running drought conditions covering much of the United States are playing any roles. We may never know the cause of these observations, but will be on high alert next year for similar trends.
By Robyn Perkins, LSLBO Bander-in-Charge