Why do tree swallows include large feathers when building a nest? Why don’t other species do this?
Placement, construction, shape and material are essential nest considerations. Each species meets its own needs with its own style, developed over eons.
Many bird species employ feathers in their nests. Mostly, small feathers are added as a finishing touch for nest walls and floor, like wallcovering or carpet.
Tree swallows, however, use large poultry feathers, as many as dozens. The feathers enclose the grass nest base. Why?
I can do research close at hand. I tend about three dozen nesting boxes, some in our yard, others at a nearby golf course.
It is easy to know the species using the box — just look at the nest.
Chickadees build an inch-deep cushion of moss. They find moss, which is not necessarily easy to do. (Go outside. Find some moss.) They pluck bits of moss, making many trips to do what instinct demands.
House wrens weave sticks and twigs, forming a chute ending in a bowl nest. The male wren, a craftsman, wedges and bends his construction material to create tension that holds everything tightly together. He will begin construction on more than one nest, finishing the one chosen by his mate.
Bluebirds build a bowl of soft grass, sometimes using pine needles. These are the plainest of nests.
Tree swallows — and this causes my wonder — always incorporate large feathers in their nests.
The swallows — usually several pairs on the golf course — will find and use perhaps a dozen feathers each. How many flight miles does it take to find them? If a bird returns to nest in last year’s box, does it remember its feather source? Do swallows share sources?
Feathers are valuable enough to cause fights between males. A research paper said a pile of feathers near nesting sites will cause a “frenzy.” (I’m collecting feathers this season, to be scattered near nest boxes. I want to watch this.)
Biologists say that feathers in tree swallow nests provide survival insurance — insulation and protection from parasites.
Feathers keep temperatures higher inside nest boxes. This is thought to be important when the adults, in cold spring weather, have to choose between brooding chicks or taking extra time to hunt for food.
Tree swallows are aerial feeders, capturing all their food on the wing. A day or two of rainy weather at the wrong time can put both parents on the hunt, leaving eggs or chicks uncovered.
Research has found a correlation between the number of feathers in a nest and a shortening of the time it takes the brood to fledge. Feathered nests produced larger chicks that leave the nest sooner.
Chicks in nests lined with feathers had lower infestations of lice and mites, according to the results of one study. Parasites can hinder growth.
Chickadees, wrens and bluebirds all use my nest boxes, the same style box, often no more than 10 feet apart. Those three species don’t feather their nests. Neither are they aerial feeders. They pluck insects from trees, shrubs and the ground. Weather is less of a factor.
Is that the key?
Given the other benefits, however — chick growth, parasites — why not use feathers?
Is the difference simply the problem that inclement weather can create? Are the growth benefits for the chicks just a bonus? How did this get started?
I cannot find answers. Birds are complicated creatures. And they don’t talk.
Read Jim Williams’ birding blog at startribune.com/wingnut.