Field Observation 4: Migration

On April 4th I went down to Lake Champlain to explore some small, fragmented woody areas near the Echo center. I also looked for birds in the bushes around a grassy park. The most striking birds I spotted were a large flock of Common Grackles in one of the small wooded areas. There was a group of at least 20 of them, though they tended to fly away too quickly for me to get a good picture of more than a couple at a time. According to All About Birds, our location is just barely on the border between their year-round and migratory summer breeding range. These birds are loose colonial nesters, though to my understanding do not necessarily flock unless they are migrating, and wintering colonies/foraging groups tend to be mixed flocks of different blackbird species. From this I assume that this group was a collection of short distant migrants. According to Birds of North America, Common Grackles that breed at latitudes of 44-45 degrees N tend to migrate about 1000 km, assuming that their path is direct and primarily from South to North, that would put this groups wintering location at somewhere around North Carolina, well within their year round range. As for why some Common Grackles migrate, while others from the same area may not may be due to resource competition. As more northern areas come in to spring, new untapped resources come into season, and moving into these areas would mean less competition from others during the breeding season. Grackles from even more southern populations such as those along the Gulf Coast generally are wholly non-migratory, likely due to a considerably higher abundance of resources available year round.

Another migratory bird I saw a handful of were American Robins. Here in Burlington, we are well within the year round range of these birds, and I have seen them throughout the winter, though they do certainly seem to be more common lately. According to Birds of North America, most American Robins migrate 500-1200 km between winter and summer ranges, though reduction in migratory distance to as little as 100 km has been more and more frequently observed as a result of climate change. In any case, it is relatively uncommon for the American Robin to overwinter in this part of Vermont, and assuming that these individuals are of the median standard migratory distance, that would put their wintering location somewhere around Maryland. Robins are ground feeders, relying heavily on soft bodied insects that would mostly be inaccessible in the frozen grounds of a Vermont winter, and like the Common Grackle likely migrate to evade competition from other birds and take advantage of newly accessible resources farther north.

Posted on April 9, 2018 03:39 PM by