Migration

Date: 4/3/19
Time: 8:00–9:00AM
Location: Centennial Woods, Burlington, VT
Weather: Chilly, around 32 degrees, warming up to 37 in sunny spots
Habitat: Dense mature forest of Eastern Hemlock, edge habitat composed of deciduous trees and shrubs, and open meadow with phragmites, a creek, and Staghorn Sumac.

I observed an American Robin singing an unusual version of his song high up in a tree. It sounded like a Robin's voice and had all the right notes, but they were put together in a strange order. I visually identified it as a Robin through binoculars. I wonder if this was a juvenile undergoing the song crystallization phase, trying out his voice and practicing adult song without quite getting it right yet, or just a rare form of the song that I hadn't heard before (see recording below). American Robins overwinter in Vermont, but have only starting singing territorial songs within the past several weeks. Being generalists, it is more cost-effective for them to find food here year-round than to invest a lot of energy resources in migrating to an area where the food might not even be much more available than where they were to start out with. In the summer, they can eat insects and grubs, fruit, and seeds. In the winter they can find frozen bugs in trees, fruit like crab apples and pin cherries that cling to the branches all season, and maybe even some nuts here and there. Thriving in suburbia, they might take advantage of bird feeders or anthropogenic food. Their large bodies are well-adapted to surviving the cold, with their plump shape minimizing surface area and maximizing retention of thermal mass. Dark-eyed Juncos also stick around all winter, and have similar body shapes and diets to Robins. Juncos huddle in cavities overnight for warmth in the winter, but I wonder where Robins roost when it's very cold.

In a treetop near the Robin, I heard chatter and saw smooth sleek brown birds. They were identified by Michael MacDonald (ornithology professor at UVM) as Bohemian Waxwings, facultative migrants who are passing through Vermont on the way to their breeding territories. According to the Cornell Lab of Ornithology, Bohemian Waxwings overwinter in the middle of the US, and travel up to Northern Canada and Alaska for breeding. They appear at our latitude for just a brief period in early spring, stopping over long enough to refuel. Their photosensitivity at this time of year produces hormones triggered by increasing daylight, which allows them to match their physiology with the environment so they don't fly north too early. They were taking advantage of the abnormal abundance of crab apples in the area, which had a great reproductive year and overwhelmed the consumption needs of local species.

Using the range maps and migration information provided by All About Birds (Cornell Lab of Ornithology) I determined that the average migratory distance travelled by the birds I observed totaled up to more than 12,530 miles! To put that into perspective, that's about twice the distance from Oregon, US to Santiago, Chile! Even the distance traveled by individual birds is very impressive – some Mourning Doves and Chipping Sparrows likely traveled over 2,500 miles from Southern Mexico up to Vermont and other Northeastern states.

由使用者 zoey_november 在 2019年04月08日 20:55 所貼文