As March winds down, and April approaches, our thoughts turn to Tree Swallows (Tachycineta bicolor), soon to return to southern Ontario to dazzle us with their brilliance.
These entertaining hirundines exhibit enough varied behaviour, with both predictability and the possibility of the unusual, to keep us occupied and entertained all day.
There are many aspects of Tree Swallow behaviour that defy immediate explanation, one of which is the preference for white feathers for the nest.
In a fascinating book, (White Feathers, The Nesting Lives of Tree Swallows) by that most consummate of naturalists and authors, Bernd Heinrich, an interesting hypothesis is advanced.
It is an established fact that some level of egg dumping occurs with Tree Swallows, meaning that a female not having a nest of her own, or for some other reason, will lay her egg in the nest of another pair. This action can only be successful if the resident female has already started to lay eggs, otherwise a strange egg suddenly appearing would be instantly apparent, and rejected. Furthermore, once the rightful occupant of the nest has laid even one egg, she would no longer be able to differentiate her own egg from the parasite egg since they are identical. It is postulated that a canopy of white feathers camouflages eggs in the nest, so the would-be dumper looking in would be unable to see the existing clutch.
Francine Gilbert took several pictures of a swallow carrying a white feather, finally angling it to get it into the nest.
In contrast with the male's fabulous gleaming plumage the female is quite drab, shown here peering from a nest box.
This nest is amply lined with feathers, but not all are white, or even pale-coloured.
One must conclude that white feathers are gathered preferentially, but other feathers are used when nothing else is available. Heinrich found that white feathers were so desirable, especially those in the 8 - 10 cm range, that the male swallow would take them right from his hand.
The base of the nest is always made of dried grass as is shown below.
The feathers are added at the end, with the point facing downwards and anchored into the substrate forming a canopy over the eggs.
In addition to functioning as camouflage the covering of feathers probably also has a thermo-regulatory function.
Here is a female at the lip of the nest hole, with a male flying by.
It is an accepted fact that cavities are in short supply, with many species competing for them, so it has always been a puzzle to me that every year at SpruceHaven, perfectly good nest boxes remain unused, while others, identical in every way, become home to Tree Swallows, Eastern Bluebirds (Silia sialis), House Wrens (Troglodytes aedon) and Black-capped Chickadees (Poecile atricapillus).
Heinrich makes the interesting observation, "I now had the hunch that the swallows were protecting sky space rather than one nest box or another." Why this would deter other species, however, remains unclear.
Males are not usually tolerant of each other during the breeding season, but here is an interesting example of two males seemingly interested in the same nest box, without antagonism - at least none that we observed.
A mated pair is more to be expected.
Here are a few more pictures of this delightful bird, the first of a male and the second and third of females.
The goal of all species is to pass on their genes and here is the result of a successful Tree Swallow season, as recent fledglings gather to practice their flight and food-gathering skills.
They have a long and arduous journey ahead of them, with many dangers along the way, to winter in Central America, primarily in Honduras, with some birds even going as far as northern South America.
Many will not make it, but those who do will set out to return to their place of birth the following spring.
As always, we will be here to welcome them and provide a snug nest box they can call home. It's the least we can do.
