Monitoring Tree Swallow Nests

Nature Notes for June 24, 2011

Over the past two weeks, I have been spending a lot of time with Tree Swallows. For about 15 years, I have been monitoring their nest boxes and banding the young.

Tree Swallows females are quite synchronous in their egg laying. That is, all females in an area will lay their eggs at almost the same time and start incubating at the same time. That means that I am very busy in mid-June. This year, chicks were ready to band starting on June 8 and almost all have been banded as I write. Many young birds have fledged already. There are only a few nests that still have nestlings in them.

One of six species of swallow that nest locally, Tree Swallows are cavity nesters. Prior to European settlement, they used old woodpecker holes, often in dead snags around beaver ponds. However, like the bluebird, they nest readily in man made nest boxes. This has made them a good species to study, since the nests can be found easily and Tree Swallows are quite tolerant of disturbance.

Tree Swallows nest throughout most of North America. They winter from the Gulf coast to Central America. They can winter further north than most swallows since they are not completely dependent on flying insects for food. They can digest the berries of the wax myrtle, a common coastal shrub.

The plumage of adult Tree Swallows is iridescent blue/green above and white underneath. They have a slightly forked tail, but not the long tail streamers of a Barn Swallow. Female Tree Swallows take two years to acquire their iridescent plumage. This is unusual for female songbirds. In their first breeding year, they are primarily brown above, with sometimes a few scattered green feathers.

Nests of Tree Swallows are made of a neat cup of grass lined with finer grasses. If pine needles are available, they may make a nest of them instead of grass. They also always contain feathers which act as insulation for the chicks. I have found body feathers of ducks, gulls, Ruffed Grouse, Wild Turkey and domestic chicken lining Tree Swallow nests.

Tree Swallows lay small white eggs, typically five or six to a clutch. When the female is laying, she lays one egg per day until the clutch is complete. Only then does she start incubating. Sometimes incubation does not start immediately. Incubation can be delayed for up to two weeks and the eggs will still be viable.

Incubation typically takes about 14 days, with the entire clutch hatching on the same day. Newly hatched chicks are naked and helpless. They seem all mouth and stomach with very truncated wings and legs. It hardly seems possible that in three weeks they will grow enough to have feathers and be able to fly. “Eating like a bird” takes on a whole new meaning when you see how quickly they grow. It takes a lot of flying insects to fuel that growth.

Although at the beginning of the nestling period, the adults remove the fecal sacs from the nest, as the chicks grow, they stop doing this. The result is that by the time the chicks fledge, the nest is a matted mess which needs to be scraped from the box. Successful Tree Swallow nests, that is chicks survived to leave the nest, are disgustingly filthy.

Although there are a few exceptions, most Tree Swallows have only one brood per year.

So far this year, I have captured 3 previously banded adult females. One was banded as a chick in 2007, one as a chick in 2008, and the third as an adult in 2008. The third bird is at least 5 years old, although she could be older. The oldest known Tree Swallow on record is a male which hatched on a long-term study site at Long Point on Lake Erie. This bird was a little over 12 years old when last captured, alive and well and at an active nest on Long Point.

The last Breeding Bird Atlas for Ontario reported a significant population decline in Tree Swallows since the preceding Atlas, as with all aerial foragers. The reasons for this decline are probably many and include habitat loss, drainage of wetlands, changes in farming practices, competition with other species for nest cavities and pesticide use which reduces their food source.

Since this species is quite tolerant of humans, it is really unfortunate that human activity is the main cause of its decline.