UVM Ornithology Field Journal 5: Migration
Date - 4/4/21
Start time - 10am
End time - 11:30 am
Location - North St. and surrounding areas, Winooski
Weather (temperature, wind speed/direction, precipitation) - 52 degrees Fahrenheit, 13 mph wind, 60% cloud cover, no precipitation.
Habitat(s) - Residential/suburban area with sparse trees/little vegetation.
Year-round species I saw were Black-capped Chickadees, European Starlings, Ring-billed Gulls, Mourning Doves, Northern Cardinals, and American Crows. Foregoing migration may allow birds to avoid the risks inherent in migration (predation risk, etc.) while maintaining their own territories and also enabling them to capitalize on spring resources the moment they are available (VS. migrants, which must stake a new territory claim every season and may arrive after resources have begun to become available).
However, overwintering itself requires a number of adaptations to the cold, both behavioral and physiological/morphological, which can be costly - Black-capped Chickadees, as a case study, roost overnight in cavities to keep warm, have extra insulating down feathers, cache foods to save for later (something our overwintering corvids will also do), and have fairly generalist diets which may aid in their ability to survive in the low-resource environment. Species dependent on insects are absent from VT in the winter. Shivering, cross-current exchange and regulated hypothermia are other ways in which overwintering species cope with the cold.
The obligate migrants I saw were the Eastern Phoebe, Turkey Vulture, and Common Grackle. Common Grackles typically move further into the western and southern portions of their ranges in the U.S. during the winter, but some may stay in far southern VT, so they would be coming from no less than 100 miles away, but most likely further. Turkey Vultures in the northeast winter in the southern states usually, ~850 miles away. Eastern Phoebes migrate similar distances. That adds up to a very approximate distance of ~1,800 miles migrated total by these 3 species to get to Vermont. Arriving in early April before other migrants may allow these early migrants to stake out territory claims and get a headstart on breeding---but, as the last week or so has shown us, early spring can be very variable in terms of temperature and conditions, so it's a gamble, and enduring snowstorms may be especially challenging for some species not adapted to overwintering.
As for facultative migrants, until recently we had Pine Grosbeaks and other irruptive species in VT which had come due to low food availability in their typical, more northern ranges to eat shriveled fruits and seeds. These seem to have departed at this point, going back to their northern ranges as winter turns to spring and food becomes more available. I saw European Starlings, which I believe will either stay in their northern range here in VT year-round or migrate south to the middle states; the decision is probably based on food availability, the harshness of the winter, and maybe on dominance status, too. The environment can support fewer individuals over the winter due to lower food availability, so dominant individuals may stay while individuals in excess of the carrying capacity for that species in VT winter may be forced to migrate.
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