Tuesday, May 15, 2012

Tree Cavity Nesting and the Black-capped Chickadee

Black-capped Chickadee nestlings
Some cavity nesting birds are fully capable of excavating their own nest holes and some aren’t. Those that can’t do the work themselves must rely on finding natural tree cavities, or more typically, on finding holes that other birds have already excavated. Woodpeckers play a crucial role in creating shelter and breeding opportunities for scores of other animals, including many mammals. Of course the woodpeckers aren’t doing this for altruistic reasons, and they are not paid contractors either (How precisely do you pay a woodpecker? Perhaps in monthly installments of beetle grubs?) Obviously, the woodpeckers are building for themselves, but when they are done nesting, their house immediately goes on the market where it will be snatched up by another animal. Given a healthy supply of both trees and woodpeckers, a habitat can be replete with cavity housing.

Flickers commonly create cavity nests that other birds and squirrels will reuse

As far as tree cavities go, it’s not one-size-fits-all. Some birds are very particular about the size of the entrance hole and the dimensions of the cavity itself, while other birds aren’t so picky  and will accept cavities and entrance holes of various shapes and sizes. A few years ago, we had a White-breasted Nuthatch nesting in a spacious Screech Owl nest box. I guess that this nuthatch family enjoyed the high ceilings.
A White-breasted Nuthatch enjoys the view from his "mansion"
Birds in our area that cannot excavate their own nests cavities include: Eastern Bluebird, Great-crested Flycatcher, Wood Duck, Hooded Merganser, Common Merganser, House Wren, Tree Swallow, Chimney Swift, Eastern Screech Owl and Saw Whet Owl. For the most part, all of these species are relying on the kindness of strangers –meaning woodpeckers and people, to provide them with housing. Some birds like the Nuthatches, Titmice and Chickadees, most often use former woodpecker holes, but they can also make their own nest cavities.

A Chickadee with a beakfull of wood chips
The Chickadee's soft nest made largely from animal hair and moss
I recently watched a pair of Black-capped Chickadees doing just this –and it was a slow process. For their tree they selected a broken off trunk of an Eastern Hop Hornbeam. This was an interesting choice, since the wood of the hornbeam is very dense, and I imagined that excavating it would be much more laborious than if they had chosen virtually any other kind of tree. I think that what clinched it for them was that a woodpecker had already chiseled out a starter hole for them (woodpecker contractors probably don’t charge too much for this service.) Also, I suspect that the tree’s heartwood was somewhat rotted, and so the wood was likely soft enough for a chickadees’ small beak to deal with.

Red-bellied Woodpecker reuses a nest made by another woodpecker
The Chickadee pair worked on their excavation for at least several days. They took turns pounding away at the wood and biting off small beak-fulls of sawdust, which they would then release into the wind while flying to a nearby perch. When the interior of the hole was large enough, the female began to bring in soft nesting material –consisting mostly of moss and animal hair. Inside the cavity, the chickadee nest consists of a thick mattress of moss that covers the entire floor of the cavity. On top of the moss layer, a “comforter” made primarily of animal hair is laid down. The eggs are laid in a small cup-shaped depression in the middle of the comforter.
Another Chickadee pair is nesting in one of our Bluebird boxes. 7 white eggs with reddish brown spots were laid into the nest and incubated by the female. It is thought that cavity nesting may be a relatively recent adaptation for Chickadees and Nuthatches, since their eggs retain colored spots consistent with the need for camouflage. Typically, birds that nest in dark cavities have no need to produce eggs that blend into their surroundings, and so most of them lay eggs that are white and/or have no spots.
The young chickadee nestlings hatched just this week, and now both parents are engaged in feeding duties.

1 comment:

  1. Hello, I work for an organization called Scientists in School(scientistsinschool.ca). We do a Bird day camp program and need a picture of a chicadee nest to show its location. I wondered if we could use the image in the tree trunk up above. If so, would you mind sending us the original.
    We will make sure your name is on the photo!!
    Anne Purvis