Snags: Nature’s apartment complexes

Photo © James Butler

Most people know exactly what a snag is, even if don’t know it by that name. A snag is simply a standing dead or partially-dead tree. While some people may view them as hazardous or untidy, snags are actually quite important to wildlife. Many animals make their homes in or on snags, including birds, bats, small mammals, snakes, and invertebrates. Snags are extremely important structural elements in forested habitats, and may make up the entire habitat of some creatures, like insects. Snags are so important to birds that here at Sapsucker Woods, there are actually a few “imports” salvaged from other locations which are “planted” in the ponds here; they quickly became the favored nesting spots of our Great Blue Herons in summer and a preferred raptor perch in the winter! There is always something happening at the snags.

Nature’s Apartment Buildings

A hole in a tree is called a cavity; it can be caused by decay, lightning, woodpecker activity, or an injury to the tree. There are about 80 species of birds in North America that nest only in cavities, from the mighty chickadee to the marvelous Wood Duck. Two kinds of birds nest in cavities: primary cavity-nesters and secondary cavity-nesters. Primary cavity-nesters excavate their own nesting hole, like most woodpeckers. Male woodpeckers drum on hollow snags to claim territories and win females. Once a pair is established, some woodpeckers need at least four cavities: one for nesting, two for separate male and female roosting places, and another for the roosting fledglings.

Prothonotary Warbler

Prothonotary Warblers are one of only two warbler species that nest in cavities.

Photo © Tim Lenz

Secondary cavity-nesters do not excavate their own nesting holes but depend on other species to do the hard work. Many species of birds, spanning a staggering array of families (flycatchers, titmice, nuthatches, raptors, bluebirds, and so many more!) just cannot reproduce without these cavities. If you have put up a nest box, you may have noticed that during the breeding season it becomes prime real estate, and what is a nest box but a simulated cavity? To start a natural cavity, find a tree whose heartwood is rotten and drill a 2″-diameter hole into the heartwood about 3″ below a stout limb. This is a great entry point for a chickadee to begin excavating the rotten wood! All the better if you can do this on a downward-facing side of a tree to keep the rain out.

In cold-climate regions, preferred nesting cavities usually are oriented South or East to maximize their sun exposure. In warmer climates, preferred cavities are oriented North to reduce overheating.
Acorn Woodpecker

Acorn Woodpeckers maintain and defend a communal stash of acorns that they cache in decaying snags or posts called a granary tree.

Photo © Nature Kat

Dead Wood Shoulds

Certain forestry practices and land use legacies have left many young forests and fields without enough
snags to meet the needs of birds. Here are some things you can do to manage for cavity-nesters:

  • Where habitat is suitable, you can install nest boxes for cavity-nesting birds. Keep in mind, however, that while nest boxes do provide more nesting opportunities for secondary cavity-nesters, they cannot totally replace the wildlife value of an intact snag.
  • If you are considering a timber harvest on your property, talk to an experienced forester about leaving a few important wildlife trees and snags intact. Keep in mind that the bigger the better, so leave big trees when you can.
  • If you have several acres and can spare a few poorly-formed or short-lived trees, consider girdling a few trees >12″ in diameter per acre to create more snags. Candidate trees should have a gradient of rot resistance, ranging from trees that will rot quickly (e.g., willow, ash, maple, pine, hickory) to those that will take longer to decay (e.g., juniper, black cherry, black walnut, black locust, white oak); in this way, you will be managing for snags into the future.
  • Where existing snags do not threaten people or property, simply leaving them in place until they fall on their own is all you need to do.
Wood Duck and Painted Turtle

Once they fall, snags are used by a different set of species.

Photo © Gerry Sibell

Coarse Woody Debris

Even once they have fallen, snags continue to provide cover or basking sites to amphibians, turtles, small mammals, and, of course, birds. The male Ruffed Grouse, for example, uses hollow logs as drumming structures to advertise their vitality to potential mates (see it here). Like woodpeckers, drumming is their primary means of sexual advertisement, and they depend on logs to amplify this display. Other birds prefer to forage on fallen logs, like Prothonotary Warbler, Connecticut Warbler, Winter Wren, and others. Don’t remove fallen logs if you can help it; relocate them nearby if they fall in an inconvenient location.

Snags

Photo © Cornell Lab of Ornithology

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