Appearance of tree swallows provides glimpse into behavior of nestlings

As do many plans, my plan to install nest boxes throughout my yard fell a little short. We only got one put up – for tree swallows – and late in the season at that. I figured I had lost my chance to attract nesting birds to the box and would have to wait for next spring. Not so!

It was after the first rainy, cool spell earlier in the season. Within a week of putting up the nest box, a pair of tree swallows had claimed it as their own and began gathering nesting material. They’d alight and tug bits of grass from the ground, making several trips an hour. It was wonderful to observe their activity and to have them in such close proximity, to enjoy their electric-blue plumage and liquid songs trickling down out of the sky.

I knew tree swallows incubate their eggs for approximately two weeks, and waited expectantly for the big event. I was thrilled one day when I finally heard the nestlings’ begging calls coming from the box. The adults began carrying the nestlings’ fecal sacs from the nest, which keeps it relatively sanitary early during this phase. The eggshells may also be discarded but are sometimes eaten for the much-needed calcium they provide.

When the young fledge, after about 21 days, they will be easy to distinguish from their parents: their upper-body feathers will be gray, and their chests and bellies will be dull white, with a dirty brown band extending across their chests. This is in stark contrast to the blue-green metallic sheen of the adult’s upper-body plumage and stark, clean underparts.

I hope to be present for this event, because once the young leave the nest cavity, they generally don’t return, unless it is at night for a few days afterward. Their parents will feed them for at least three days.

Because tree swallows often nest near or in human-made structures, much attention is paid to their comings and goings. People often remark on the apparent disappearance of swallows from an area, and this is a well-known occurrence that has been documented in scientific literature. Why they do this or where they go is unknown, but it seems to correspond with cool and rainy periods, and may occur even in the middle of incubation. Surprisingly, this has little effect on the eggs. The birds may be gone for up to a week, but once they return incubation is resumed without incident (assuming the weather hasn’t been severe).

Tree swallows are better able to handle inclement weather than other swallows because they eat fruit (such as bayberries) and seeds of bulrushes and sedges, as well as flying insects. This trait also allows them to winter farther north than other swallows.

NEWS bird columnist Chris Corio can be reached at bdnsports@bangordailynews.net