Cavity Lore

March 2006
by Sam Crowe

According to James Tanner, who studied Ivory-billed Woodpeckers extensively during the 1930s, ivory-bills had a typical foraging range of up to four miles (6.4 kilometers). The range tended to be smaller during the nesting season (January to July), and expanding later in the year. The current search areas are long and narrow, with a wide range of tree types. Researchers speculate that under such circumstances an ivory-bill's foraging range might stretch as far as 15 or 20 kilometers.  This is a lot of territory to cover, especially in heavily wooded and isolated terrain. In order to improve the odds of locating an ivory-bill, the search team's focus includes locating possible ivory-bill roost locations.
Eight woodpecker species can be found in the Big Woods during the winter months.  They produce a lot of roost and nest cavities of different shapes and sizes.

An ivory-bill may have several roost holes throughout its foraging range. It may  return to the same location several nights in a row or move to other locations in an irregular travel and roosting pattern.  One of the jobs of the full-time searchers is to try to identify potential ivory-bill roost locations and to then set up a time-lapse camera to monitor the promising holes. With luck one of the cavities on the “trap line’ will be a part of an ivory-bill’s route.

The biggest challenge, after finding cavities of interest to begin with, is in differentiating Ivory-billed Woodpecker holes from those created by Pileated Woodpeckers.

Before learning more about identifying ivory-bill cavities…

Here are answers from a few of our readers…and our free personality analysis.

1. Spilled ink (dull, unimaginative)

2.  Packaging for Gateway Computers (cheap, lonely)

3. Cows (milk lover)

4. Black holes into parallel universes (scientific nerd)

5. Pileated Woodpecker hole on the left, Ivory-billed Woodpecker hole on the right (birding savant – if this was your answer give us a call.)

As an identification aid, each searcher carries a card, which looks very similar to the image above, showing a typical size and shape for both pileated and ivory-bill cavities.

An ivory-bill cavity is typically somewhat oval and irregular in shape, approximately 4.3 by 5.4 inches in size.   Pileated cavities are usually round to slightly oval in shape, about 3 by 3.5 inches in size.

Pileated Woodpeckers also create rectangular feeding holes. These cavities can be as much as 12 inches in length and multiple cavities can sometimes be observed on a single tree.

Audubon’s observations may offer other clues in identifying ivory-bill cavities:  “The birds pay great regard to the particular situation of the tree, and the inclination of its trunk; first because they prefer retirement, and again, because they are anxious to secure the aperture against the access of water during beating rains. To prevent such a calamity, the hole is generally dug immediately under the junction of a large branch with the trunk.”  (J. J. Audubon 1842. Birds of America. Vol. 4. J. B. Chevalier, Philadelphia, PA. Reprinted 1967, Dover Publ., New York.)

In contrast to this, and adding to the mystery surrounding this bird,  James Tanner reported that few of the roost or nest cavities he observed had any special protection from the weather. (James T. Tanner, The Ivory-Billed Woodpecker, 1942)

Another clue for identifying an ivory-bill cavity comes from F.M. Phelps: “Ivory-billeds not only stripped bark from trees during foraging, but also seemed to characteristically strip bark for a few meters below their nests.” (Phelps, F.M 1914. The resident bird life of the Big Cypress Swamp region. Wilson Bull. 26: 86–101.)

A combination of the above reference points, plus field experience, are used to determine which cavities deserve further study.

The search teams expend a lot of time and effort in trying to locate cavities created by an Ivory-billed Woodpecker, a possible key element in eventually understanding more about their range and population in the Big Woods section of Arkansas.