Thursday, April 4, 2013

Tree Swallows Have Arrived

Tree Swallow (Tachycineta bicolor) copyright John Ashley
Tree Swallow (Tachycineta bicolor) observed yesterday in northwestern Montana

Tree Swallows (Tachycineta bicolor) have returned to western Montana, about a week earlier than average. Of the nine North American swallow species, this one winters further north and returns to its breeding grounds earlier than the rest.

Tree Swallows nest all across Montana, faithfully returning each spring to their previous breeding sites. One study found that 96% of adult males and 86% of adult females returned to the previous year's nest. These swallows like to nest near open water, where they forage for flying insects. But these early-returning birds won't start egg laying until the flying insect populations reach a certain density, ensuring enough food for their hungry hatchlings. Insect populations are in turn related to average air temperatures and, as our climate warms, Tree Swallows have moved their mean date of nest-initiation forward by 10 days over the past 30 years.

Researchers also found a correlation between the timing of egg laying and mean wing beat frequency, as female Tree Swallows with faster wings tended to start laying eggs earlier than "slower" females. Faster wings probably translates to more efficient foraging, which in turn would speed up the accumulation of springtime fat deposits that are necessary for producing eggs.

It's hard not to just sit by a nest box and watch these aerial acrobats for hours. But they're even more amazing on the southern wintering grounds. Tree Swallows are more sociable in winter, when many thousands gather into huge, swirling flocks about an hour before sunset, eventually landing to spend the night at a communal roost.

If you're interested in providing nest boxes for Tree Swallows, you can find all manner of nesting and life history information here.