Tree Swallows Have Arrived

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.

Waiting for Groceries

Juvenile Cliff Swallow waiting for a food delivery

Apparently, I've never paid close attention to Cliff Swallows before. But while working around an old horse stables last weekend,   I couldn't help but stop and watch the swirling clouds above our heads. The adult swallows poured in and out of their mud-ball nests on three sides of the building.

The adults flew in dizzying circles, delivering delicious insects to their nearly-grown chicks. The juveniles watched too, from the opening in each active nest, waiting for the next parent to bring yet another taste treat. This working stables wore a necklace of several dozen swallow nests under each eve. It seemed like a lot of birds to me, but the largest Cliff Swallow colonies in the west can number 3,700 nests or more.

Young Cliff Swallows born in Montana will soon fledge from their nests and form creches with other juveniles. By August almost all of them will be winging their way towards South America, where they'll spend the winter. Why migrate so early? Because they only eat live insects, so they have to leave Montana before an early frost depletes their groceries.