Mitton: Extremely social tree swallows

Daily Camera, CO – Apr 3, 2014
By Jeff Mitton (Contact)
While camping in Brown’s Canyon on the Arkansas River, I was entertained at the end of the day by a large population of tree swallows dining on the wing, or snagging flying insects.

Their aerial acrobatics were a pleasant distraction for me, but this was hard work for them; they were foraging to feed a large number of hungry mouths back at the nest.
The tree swallow, Tachycineta bicolor, is 5 to 6 inches long with a wingspan of 12 to 14 inches. Its back is an iridescent greenish-blue, its head and wings are brown, and its belly and throat are white. It can be distinguished from the violet-green swallow by the eye, which is surrounded in white in violet-green swallows but brown in tree swallows.
Tree swallows overwinter near the Gulf Coast and other areas around the Caribbean and nest as far north as tree line in Canada and Alaska. These swallows are highly social, are known for nesting in large congregations and for dense flocks that swirl and swoop at dusk.
Nesting records from 1959 through 1991 indicate that tree swallows are responding to the warming climate by laying eggs at least nine days earlier in spring.
Tree swallows are monogamous, at least in the sense that a male and a female tend a nest and feed the offspring.
But only a small percentage of nests contain chicks that were all sired by the attending male. The majority of nests contain chicks sired by promiscuous or extrapair matings, and the percentage of chicks sired by extrapair matings varies from 50 percent to 92 percent. Some nests have chicks sired by the attending male plus chicks sired by two or more other males.
Females can accept or reject copulations so the female is not a victim of philandering males; she is complicit in seeking additional sires for her chicks.
The selective advantages of extrapair matings were revealed by a genetic study that identified broods with or without chicks from extrapair matings. Higher proportions of eggs hatch and higher proportions of chicks fledge from nests containing extrapair matings. Swallows seem to have some genetic incompatibilities, so females enhance their reproductive success by sampling a variety of males.
The genetic study was able to identify some of the males that sired extrapair chicks. Nests of these adventuresome males contained chicks sired by others. Turnabout is fair play.
Females lay from two to eight eggs, with an average of 5.5 eggs. All of the eggs in a small clutch might hatch in the same day, but the majority of nests and all of the nests with many eggs have chicks hatching out one to three days apart. The asynchronous hatching produces a great disparity in size; by the time the last chick hatches, some of its sibs may weigh twice as much. This initial size advantage carries over to larger size at fledging and larger adult size. The chicks that hatched late have the highest mortality, or the lowest probability of fledging.
When a male arrives at his nest to share food, he sees a heterogeneous batch of chicks; some are his, some are not, and some hatched so late and are so small that they will probably not survive.