FJ3: Ecological Physiology
The American Robin, House Sparrow, Black-Capped Chickadee, Northern Cardinal and American Crow all use aspects of evolution and ecology to survive in the wintertime. During my field observation, I noticed around 15 American Robins all in one tree. They were all eating berries and seemed to be getting along just fine. I returned to the same spot about 20 minutes later and there were no birds to be found. I believe they were gathered around this one tree because their resource distribution is pretty clumped in the winter. Burlington only has so many fruiting trees and this tree was the only one on the street with berries/birds on it. Normally, American Robins can spread out relatively evenly, foraging for worms and insects, but those creatures are much harder to find when it starts getting cold. Additionally, they were practicing cooperative feeding, a social strategy that can increase foraging efficiency, protection from and awareness of predators, as well as create some agonistic encounters between individuals.
Black-capped chickadees are a commonly seen backyard bird, particularly during the cold months. I saw around 5 individuals on my walk. They carefully hide certain food items, have dense winter coats, specially selected winter roost cavities, and the ability to go into nightly hypothermia, helping conserve large amounts of energy and increasing the chances of survival. They eat enough seeds and frozen insects to gain an additional 10% or more of their body weight each day and use this excess body fat to shiver all night to keep warm. Birches provide good roosting conditions for them and help insulate them at night when temperatures really plummet.
Torpor, hypothermia, and countercurrent exchange are all tools that birds can use to stay warm in the winter and retain body heat. Many of the species that I noticed on my walk had fluffed up their feathers in order to trap air underneath those feathers and warm that air. Many birds also stand on one leg at a time while the other is tucked up warmly in its feathers. Some birds bury their beaks into their feathers to prevent evaporative cooling that occurs when a bird breaths out of its nares. Birds that can’t handle the cold of the winter may migrate south and then return once the weather starts to change.
On my walk, I saw at least 30 house sparrows and 17 American Crows (they really like the dumpster in my driveway). The house sparrows were not all together but I would see them every 50 feet or so with a handful of them hanging out in shrubbery and bushes. The American Crows were either up in tree branches or they were on the ground or pavement searching for discarded food. Both of these species are scavengers that have hitched their wagons to humans very successfully. We provide year-round foraging and our structures provide them with shelter as well. I’ve seen hundreds up crows roosting together in the canopy of deciduous trees and I’ve seen house sparrows be extremely aggressive towards birds of other species, pecking and using their calls and mob mentality to ward off intruders in their territory. House sparrows are highly social birds that return repeatedly to the same hierarchically-organized flock to forage and roost during the winter. A sparrow pair can produce multiple broods in one year depending in part on environmental factors and they've already started to look for nesting sites by early November.
I saw 4 Northern Cardinals on my walk, three of which were males, and none were flocking together. Each individual was pretty far apart from the other, perched on different phone lines. In the winter, they spend their time foraging while hopping on the ground or in low bushes, readily coming to bird feeders and particularly enjoying sunflower seeds. During the winter, when insects are scarcer, they’ll eat berries and vegetable matter. All of the birds I saw on my walk were either feeding, flying, or having some sort of interaction with each other. Birds burn a lot of energy staying warm, so constant feeding helps them regulate their bodily functions.
Standing dead trees, or snags, provide birds and mammals with shelter to raise young, stay warm and other functions. Woodpeckers and creepers are the most likely to utilize snags but around 30 bird species are capable of making their own nest cavities in trees. The insulation of a tree trunk home allows wildlife to survive high summer and low winter temperature extremes. Tree cavities and loose bark are used by many animals to store their food supplies, while insects living inside the dead wood eat thousands of forest pests. When they eventually fall, fish, amphibians, and other wildlife can use this structure for protection and habitat.
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