Field Journal #4- Migration

Date: April 8, 2019
Location: Centennial Woods
Time: 1:00-2:30 pm
Weather: raining the entire time, pretty cold (low 40s), lots of cloud coverage
Habitat: Deciduous Forest

As is the case with most of my walks, I saw an abundance of Black-capped Chickadees in Centennial Woods this afternoon. Throughout my time in Vermont, I have noticed that Black-capped Chickadees have a very consistent presence throughout all of the seasons- which is why they were one of the first birds that I learned to identify. Black-capped Chickadees are largely permanent/nonmigratory residents, meaning they stay in the same general homer range throughout the entire year. There are documentations of Black-capped chickadee “irruptions” during fall months, however, in which groups may undergo irregular migration due to a factors such as habitat destruction or lack of resources. In northeastern United States, such irruptions correlate strongly with fluctuations in northern seed crops. Despite these infrequent movements, Black-capped Chickadees are permanent residents and therefore have various adaptations that allow them to survive through the fluctuating seasons of their habitat (which could be all over the country). In Vermont, the cold and long winter represent a harsh climatic environment to which chickadees have responded with several physiological and behavioral adaptations. Black-capped Chickadees undergo facultative hypothermia, for example, in which they actively decreased to a certain temperature so as to reduce heat loss and save energy. By lowering their body temperature to about 15 degrees below its normal temperature, Black-capped Chickadees can conserve a quarter of their hourly metabolic expenditure. Black-capped Chickadees also have adopted advanced spatial memory capacities which allow them to store large quantities of food for colder months in which resources may be low. They also partake in behaviors such as roosting and finding insulation during colder temperatures.

The American Robin is another permanent Vermont resident that I see quite often. This species has records of migrating slightly more than chickadees, however such movement usually entails short distances in response to food shortages, rather than temperature changes. While Robins mostly eat fruits in the winter, they switch to earthworms and insects a sa food choice in the spring. Because elevated temperatures coincides with ground thawing, it is common to see increases in American Robin numbers in early spring- however this does not necessarily indicate that they were previously migrating in a far southern location. To stay warm during winter months in Vermont, Robins often will seek shelter in dense patches of vegetation or roost with one another to share body heat. They also often are seen fluffing their feathers, which increases the amount of air next to the body and thus allows for more heat to be circulated and retained. Down feathers exist closer to their skin to trap air and insulate heat so as to keep internal organs at a safe temperature. Feathers on their underside can often fluff up to the point that they cover their legs and toes, as well. In the winter, American Robins will spend most of their day foraging and eating- which helps them to stay warm and keep caloric intake high.

White-breasted Nuthatches are also permanent residents that I have seen an abundance of this winter. Like other non migratory bird species, however, they have had irregular movements as a result of seed failure, reproductive successes, etc. White-breasted Nuthatches are able to stay in Vermont all year round by altering their dietary habits as a result of the changing seasons and available resources. While they eat insects in summer and spring months, White-breasted Nuthatches switch over to a diet of mostly nuts and seeds in colder months. Most of their circadian clocks are devoted to foraging and eating in the winter. They also have very effective ways of storing food resources and returning to them (even after months).

The last species I observed on this cold and rainy day was a house sparrow, which existed closer to the edge of Centennial woods near the apartments and buildings. House Sparrows are pretty sedentary, and usually don’t move more than a few kilometers during their lifetime. Young birds may be seen migrating somewhat longer distances in the winter time (usually from high to low elevations). House sparrows are skilled at seeking out shelter in dense vegetation or cavities/holes so as to protect themselves from harsh elements in different seasons. They also show behavioral adaptations to cold weather by huddling as well as tucking their head and feet into their feathers.

All of these birds are able to forego the migration process due to physiological and behavioral adaptations that allow them to survive the various seasons in Vermont (or wherever their respective homeland is). The exact reason for this may be that making efforts to remain in one spot is more energy efficient than spending an extended period of time flying to a less harsh climate. Also, the permanency of populations of these species most likely helps to alleviate widespread conflict over resources and ecological instability- as these birds tend to stay in their home ranges and thus maintain some sort of consistency with each other as well as their surroundings.

While the cold, rainy weather this afternoon limited the amount of bird species that I was able to observe (all of which were nonmigratory), there are several spring migrant species that are beginning to show up in the Burlington area-- exciting! There are many spring migrants that have come to VT for their breeding season (or will be very soon), including warblers, Hermit thrushes, Red-winged Blackbirds, and Scarlet Tanagers. Scarlet Tanagers, which are one of my favorites, are long distance migrators who will not typically arrive in Vermont until a little later in the season. They migrate to South America for the wintering season (usually leaving by mid/late-summer) and return back to their breeding grounds by May. Scarlet Tanagers make this long trip across the Gulf twice a year, usually traveling at night. A major incentive for species to migrate southward during the wintering season is to maintain a sustainable amount of food and resources. Scarlet Tanagers, for example, migrate to South America so that they are able to eat fruit and insects- something that would not be available had they stayed in Vermont throughout the winter. While their food source down south is suitable in the winter, they return back north for breeding season due to more optimal nest locations/opportunities. Scarlet Tanagers prefer to breed in mature deciduous forests, and build nests in oak, pine, and beech trees (among others), which are abundant in Northern areas.

While the Scarlet Tanager will not arrive until late April or early/mid May, there are many birds that are already beginning to arrive. Advantages of arriving to their breeding grounds early include being able to claim a territory and nesting spot before other competitors arrive. However, there are many instances in which birds may arrive early for breeding season and then experience an unexpected freeze. In this case, it is quite possible for ill-equipped birds to not survive the harsh conditions.

Mini Activity

Migratory distances of my observed birds (max):

Black-capped Chickadees
2 miles
Usually stays in the same vicinity during wintering and breeding seasons, however irruptive movement gives their species a 2 mile (ish) average for migration

American Robin
100 miles
The amount that a Robin migrates depends on the individual as well as the available resources. Some Robins will not migrate at all and remain in their Northern habitat, while some may fly a fairly large distance southward to access food resources (insects and fruits)

House Sparrow
No migration
Sources say that the House Sparrow maintains a pretty consistent home range all year.

White-breasted Nuthatch
1 mile
Not a lot of movement, sometimes groups or individuals will undergo irruptive migration; but usually not too far

Total
The birds that I observed have the potential to have migrated over 100 miles collectively over the past couple of months, however they are largely nonmigratory birds so that number may be high (depending on the individuals)

Publicado el abril 8, 2019 11:11 TARDE por haleyferrer