A Peek into the Domestic Life of Tree Swallows

Tree Swallows are, in my opinion, one of our most beautiful birds, with their pure white undersides and iridescent blue heads and backs.  As aerial insectivores that feed almost exclusively on flying insects, Tree Swallows are a great boon to have in the neighborhood.  Unfortunately, like many other aerial insectivores, their numbers are in decline. 

As secondary cavity nesters, Tree Swallows' struggle for survival is exacerbated not only by a decline in the insects they feed on, but a lack of the natural cavities they require for nesting as well.  Secondary cavity nesters rely on nest cavities excavated by other species (usually woodpeckers), often in standing dead trees.  The widespread practice of removing such trees in recent decades has led to a sharp decline in available nesting sites.  And Tree Swallows aren't the only species that depend on these increasingly rare nesting sites.  Western Bluebirds, Ash-throated Flycatchers, House Wrens, White-breasted Nuthatches, American Kestrels, Wood Ducks and Western Screech-owls all rely on these coveted cavities for raising their young each year as well.

Traditionally, a Tree Swallow nest cavity looks something like this:  

Or this:

However, in the absence of natural cavities, Tree Swallows will take readily to manmade nest boxes.

One of the two nest boxes documented below

I have the privilege of monitoring several such nest boxes, which were originally installed for use by Western Bluebirds but have been happily commandeered by families of Tree Swallows (and one of Ash-throated Flycatchers) instead.  

Below is the progress of two of these nest boxes throughout last year's breeding season.  Between March and June, two broods of Tree Swallows fledged from each of these nest boxes.  Eggs are incubated from 11 to 20 days before hatching, and after hatching, the young remain in the nest for 15 to 25 days.

Visits every two weeks provide a glimpse inside the domestic life of a pair of Tree Swallows, and by keeping the visits quiet, brief and infrequent, the Tree Swallows are not disturbed in their domestic affairs.  

Come take a peek!

March 13: The beginnings of Tree Swallow nest-building.
The female does the majority of the nest building, forming a cup nest of grasses and other bits of vegetation that she shapes with her body.  This may take anywhere from a few days to two weeks. 

March 27: The grass cup is lined with feathers.
The cup nest is lined with feathers (from many other species of birds) that either the male or female may collect.

April 10: Four white eggs have been laid. 
These eggs, each about 0.75 inches long, are incubated by the female for 11-20 days. 

April 24: Adult Tree Swallow with nestlings.
Tree Swallows hatch with closed eyes and featherless pink skin, entirely dependent on their parents, who both bring them food (insects).

May 8: Tree Swallow nestlings, nearly fully feathered.
Nestlings remain in the nest for 15-25 days, growing rapidly as their downy feathers develop.  These four are getting a little cramped, and will be out of the nest very soon!

Also May 8: Second nest box with its first clutch of the season.
Tree Swallow clutches typically consist of 4-7 eggs; the female lays one egg per day.  Incubation typically begins around the time when the last egg is laid, so that all of the eggs hatch at the same time.

May 22: Second nest box with a brood of seven young Tree Swallows.
These nestlings are just beginning to grow their first set of feathers, called pin feathers.

June 5: Second nest box, now empty!
Nest boxes are very messy once the brood of youngsters had fledged!  In the late summer, after all breeding activity has ceased, this box will be cleaned out in preparation for the next year.

Also June 5: A second clutch of eggs has been laid in the first nest box.
A larger, second clutch of eggs has been laid, after some refurb work is done to the nest.

June 19: The second brood in the first nest box, all crowded in!
I was worried that extreme heat waves we had in June would negatively impact this late brood, as birds can be sensitive to heat.  But after the breeding season was long finished, when I opened the boxes to clean them out, I was happy to discover that all signs indicated that this second brood of youngsters successfully fledged as well, probably by the end of June.

If you're lucky enough to see Tree Swallows twittering above grasslands and wetlands or visiting a likely nest site this spring, take a moment to admire and appreciate these special little birds that not only beautify our skies, but also do their part to keep numbers of noisome flying insects in check.