Migration
Date - 4/3/21
Start Time - 9:08am
End Time - 10:33
Location - University of Vermont Redstone Campus and Burlington Country Club
Temperature - 30 deg F
Wind 20mph NW
~100% clear
Zero precipitation
Habitats were open, grassy grounds of the Gutterson field, the swampy-pond area near the Redstone Lofts, and up past the Redstone Commons to walk the path at the Burlington Country Club. Many birds individually and in groups flitted between trees and glided on the clear air, almost in a leisurely way. The areas where we observed them were very open, leaving a clear view of their perches on fences, vegetation, and trees.
Some of the year-round species we observed were the House Finch, Black-capped Chickadee, Common Raven, Rock Pigeon, and Ring-billed Gulls, who overwinter in the Northeast and don’t need to travel far to adjust to the colder temperatures and changed food availability. Their diets primarily consist of invertebrates (e.g. worms), seeds, and fruits that still exist during the winter months. Their bills are short, stubby and even conical (like the House Finches), to better adapt to cracking open and eating seeds. In a similar vein, Gulls have a primarily piscivorous diet allowing them to hunt fish in Lake Champlain during the winter as they can in the summer.
Facultative migrants we came across were the American Robin and the House and Song Sparrows. On average the straight line distance these species travel from their wintering grounds to Burlington is ~1000-2000mi. Their migration is dependent upon the food availability of the year, and if they do decide to migrate they don’t go too far into the tropics, typically staying in the southern US range. Their diets are the most adaptable - omnivorous and generalist - often making them opportunist species. Facultative migrants can survive on the same diet as year-round species, able to take advantage of the fruits, seeds, and invertebrates available during the winter, or able to migrate down south and feed on insects if need be like the obligate migrants do. Sometimes, as with the American Robin, some members of a population stay in the Northeast during the winter while some of the population migrate because of the intraspecific competition occurring. When the more competitively advantageous population members start to feed, there is less food availability for the less competitively “fit” population members. Other times the limited food availability comes from the conditions of the year itself. The Song Sparrow is a species that we have only been starting to see on our field trips, most likely arriving from somewhere in the Southern US and coming into Burlington as the weather gets warmed and a higher variety of overall food is available, so they do not need to compete as hard to eat. This is in contrast with the American Robin, who we’ve been seeing since our first field trip and whom the majority of likely did not migrate during the winter, given the proliferation of fruit and seeds in the area.
Obligate migrants we saw were the Eastern Phoebe and Red-winged Blackbird, likely coming from tropical areas back to Burlington as the weather gets warmer. On average the straight line distance these species travel from their wintering grounds to Burlington is anywhere from ~2500-5000mi. These species are insectivores, and need to migrate down south due to the lack of insect availability in colder climates. Changing climatic conditions, however, complicates these natural circannual cycles of the different bird species. The timing of spring has been very erratic for the past few years in light of worldwide warming temperatures and anthropogenic disturbance. During some weeks the weather here is warm as though it’s on the verge of spring, and in the next week it snows. A huge disadvantage of these obligate migratory species arriving in early April is that it will continue to snow some weeks and give them a competitive disadvantage when searching for food in a climate they are not adapted to. Snow likely confuses their circannual cycles and causes a further energy deficit that was already spent from migrating back to the northeastern US. On the other hand, an potential advantage to this early arrival could be that early mating and reproduction offset this lack of initial food availability and give the birds a competitive advantage over other later arrivals for finding mates. Changing climatic conditions also makes the categories of migratory birds themselves very fluid. Tufted Titmice are omnivores but are generally non-migratory species, favoring a seed-based diet overall. However, recent changing seed availability has led them to appear further south in the US. Conversely, more and more facultative migrants are seen to be sticking around in the Northeastern US in recent years, likely due to the weather staying warmer for longer and reducing competition for those species.
Sources:
https://www.allaboutbirds.org/news/
https://journeynorth.org/tm/robin/facts_migration.html
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