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June 1998
If I were going to be a bird, I might choose to be one that nests in a tree cavity. I’ve been watching a downy woodpecker at work making a cavity in a tree across the street from my home. It seems like a really big project for such a small creature. Each species that uses cavities has particular needs. The size of the tree is important, expressed as diameter-at-breast-height, or “dbh.” This is the diameter of the tree at 4 feet above the ground. Each species has requirements for the diameter of trees it can use, and while it may use a larger tree, it will not use a smaller one. Cavities are made at varying heights above the ground, reducing competition for locations among species. The habitat around the cavity site also determines the suitability for a species, including type of vegetation, the amount of space, the presence of water.
Some species of birds, called primary excavators, excavate a new hole for nesting every year. Then there are the secondary users, birds that use old excavated cavities or use natural cavities. Cavities are usually in snags, which is the term for standing dead or dying trees. Living trees with cavities are called den trees.
A downy woodpecker spends 13 to 20 days excavating a cavity in trees that are bigger than 6 inches dbh. The nest height is 3 to 50 feet above the ground, and the female selects the site. They prefer nesting in the upper parts of dead trees set in open woodlands. They make cavities as their courtship activity and for nesting. In the fall, they make a new cavity for winter roosting, and often re-excavate the same tree year after year.
A hairy woodpecker takes an average of 20 days to make its cavity in a snag with a dbh of 10 inches or more. The nest height is 15 to 45 feet above the ground, and the male chooses the site.
The yellow-bellied sapsucker breeds but does not winter in Maine. It excavates cavities in snags, or den trees with rotten heartwood, with a dbh of 10 inches or more. It uses a variety of trees, but prefers aspen, with the nest height at 8 to 40 feet above the ground. You can see signs of a sapsucker’s presence by very small holes close together in the bark on the trunk of a tree, where it has been sucking sap. Hummingbirds, as well as hairy woodpeckers, use the sap that oozes from the holes when the sapsucker is done.
The pileated woodpecker needs between 125 and 320 acres of mature forest as habitat. Each year, it creates a nest cavity in a snag greater than 14 inches dbh. The nest height is 15 to 70 feet. In the forest, it’s easy to identify these cavities because they are long rectangles. These woodpeckers require trees with heart rot that attract carpenter ants, which is a winter staple of their diet, and while they sometimes feed at forest edges, they prefer forest interiors.
Other primary excavators in Maine include the northern flicker, which gleans ants and other insects on the ground, an unusual adaptation for woodpeckers. Sometimes a flicker excavates a cavity, other times it uses an old woodpecker hole, in trees at least 12 inches dbh. The flicker breeds in Maine, but does not winter here. The black-capped chickadee uses snags of at least 4 inches dbh if making its own cavity. It rarely uses an existing cavity, but does accept nest boxes. The boreal chickadee may excavate several holes before choosing one for a nest. Occasionally, it uses natural cavities or old woodpecker holes. The red-breasted nuthatch usually excavates its own hole, in a tree at least 12 inches dbh, but sometimes uses an old woodpecker hole.
Many birds use old woodpecker holes or natural cavities exclusively. These are the secondary users. The barred owl nests in a natural cavity in a snag with a minimum dbh of 20 inches and requires at least 250 acres of mature forest as its habitat, but prefers an even larger area. The Eastern screech owl needs trees with a minimum dbh of 12 inches. It uses natural cavities or abandoned woodpecker holes, especially those of the flicker and pileated woodpecker. It uses cavities for nesting, roosting and caching food. The white-breasted nuthatch uses natural cavities for nesting, in trees with a minimum dbh of 12 inches. Red and gray squirrels are strong competitors with white-breasted nuthatches for these natural cavities. The tufted titmouse uses a natural tree cavity or an old woodpecker hole. They do accept nest boxes, and it is believed they do not excavate their own nest sites. The brown creeper, which does not winter in Maine but does breed here, uses snags with a minimum dbh of 10 inches. It prefers to nest under a strip of loose bark, but when that is unavailable, it uses a rotted knothole or old woodpecker cavity.
Are there enough appropriate sites for new cavities, and are there enough existing cavities for the demand? Starlings and English sparrows, non-native birds, are successful competitors and often take cavities from native species. Flying squirrels and other squirrels also use cavities for nest sites, caching food and as winter dens. People often cut down snags in their yards, and in the forest snags have been considered non-productive and are removed. Snags may be dangerous if they are located in areas where we walk, and so should be removed in those cases. If they do not pose a safety hazard, we should intentionally preserve them as a necessary habitat component. For managed woodlands, there are recommendations for the number of snags to leave, or to create, including size and distribution. Reduced populations or the total loss of some species requiring cavities may result from a shortage of suitable trees. There are 58 wildlife species in Maine that use tree cavities for nesting and denning. Let’s make sure they have a home.
By Nancy Coverstone (ncstone@umext.maine.edu), University of Maine Extension Educator in Androscoggin and Sagadahoc Counties
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