Field Observation 4: Migration

I went to Shelburne Pond Natural Area on Sunday, 4/7 from 3:30 to 5:00 pm. It was a mild 50°F with little to no wind and 100% cloud cover. There was no precipitation.

I walked a ~2 mile loop around the southwestern part of Shelburne Pond. I saw two male Wood Ducks (and heard a female) in the whitecedar swamp. I heard and saw a flock of Black-Capped Chickadees in this area as well. I heard and saw at least 3 Song Sparrows in brushy edge habitat near the parking lot. I saw a Common Raven fly overhead and heard many ravens on the far side of the pond. There were Canada Geese in the fields near the access road, and also in this area I saw two American Kestrels. Initially they were sitting on telephone poles, but then both individuals flew to a dead tree in the field and subsequently flew intermittently from the tree to different spots in the field. I did not see if they were feeding. I also saw some starling and a Rock Pigeon on a nearby barn.

Black-capped Chickadees and Common Ravens are both year-round residents in VT. They are both quite smart birds. The BCCH displays both physiological and behavioral adaptations to life in winter. These include food caching and the ability to enter a state of facultative hypothermia. The raven is intensely intelligent and able to exploit many food sources as they become available. However, research still shows that a high percentage of juvenile ravens do not survive their first winter. Both birds stay here year-round because they are able to find sufficient food in the winter; generally the evolutionary decision to expend energy on migration has more to do with a lack of food than with low temperature (although availability of food depends on temperature).

The Canada Goose is a facultative migrant. It travels irregularly in response to local conditions. I'm not sure where the geese I saw were coming from, but they could have been as close as southern Vermont or northern Massachusetts. They may be staying in this area for the whole breeding season. They depend on a) open water and b) agricultural or suburban open land to breed and forage, respectively. The Shelburne Pond area currently has open water in parts of its boundaries, and the fields around the pond are thawing out and quite muddy. This likely means that there is sufficient food in the form of seeds and sedges in this area for the geese to stay, and there will be more and more food available as the grasses begin to grow and the lake thaws.

The Song Sparrow is an obligate migrant. It travels as far south as Florida and northern Mexico in the winter, and breeds in Vermont and into Canada. It has arrived in the last week or two, as far as I can tell. This "choice" must be linked to food availability. The beginning of the spring thaw must bring back to "life" sufficient insects and invertebrates for subsistence in the sparrow's preferred habitat, which is brushy edge habitat near open fields or forest edges. It's important to the Song Sparrow to arrive as soon as possible after food becomes available because the spring is when Song Sparrows establish territories and begin to attract mates. An earlier bird will tend to be a more successful bird, as long as it arrives after food has become available. However, if it arrives before food is available, it will either be a very hungry or a dead bird - a definite disadvantage.

The maximum straight line distance likely traveled by the migrant species I observed is ~ 2600 (American Kestrel) + 2300 (Wood Duck) + 1600 (Song Sparrow) + 200 (Canada Goose) miles = 6,700 miles. Wow.

Posted on April 7, 2019 10:30 PM by sam_blair