by Mike Scully
This article first appeared in the March 2006 issue of the San Antonio Audubon Society newsletter.
Native earthworms do not occur over much of the northern United States and Canada, the theory being that they were extirpated from these areas at the time of the last glaciation. Earthworms do occur in these areas today, but all are introduced species, some wreaking havoc on local ecosystems. Interesting, but perhaps a bit off-topic for a birding newsletter, unless one is writing about American Woodcocks (Scolopax minor) for whom earthworms are a matter of major import.
As much as herons are designed to capture fish, so are woodcocks designed to capture earthworms. Woodcocks are stocky and heavily built, this weight perhaps associated with the strength needed for probing the soil and for drawing earthworms forcibly from it. The long bill is stout enough to be driven into the soil, yet the tip of the upper mandible is flexible and loaded with nerve endings. The top surface of the tongue and lower surface of the upper mandible are rough, providing a sure grip on captured worms.
Of limited use in the pursuit of underground prey, the eyes of woodcocks are located very high up and in back of the skull, providing their owner with an almost uninterrupted hemispherical field of vision. The ears too are unusually located below and in front of the eyes, this odd juxtaposition of sense organs being reflected inside the skull by the unusually oriented brain.
How woodcocks locate earthworms is not well understood, the color of the soil may be important, captive woodcocks preferentially foraging in darker soil. Foraging woodcocks often walk with a wavering, uncertain gait wherein the bird sways horizontally back and forth with each step, deliberately placing and sometime stamping the feet. It is thought that such stamping and swaying may cause earthworms to move. The bird may be able to feel these movements through its deliberately placed feet and certainly hears them with its oddly located ears. In the soil, earthworms are located by touch and perhaps by the taste and odor of their mucus trails.
Wookcock breeding range extends from East Texas and North Florida north to Canada and east to Nova Scotia, and appears to be extending in parts of Canada. How this present range compares with that of previous ages is unknown, introduced earthworm species necessarily comprising the major part of their diet in the north.
Moist brush and forest edges interspersed with open areas are the preferred breeding habitat. Male woodcock display in the open, performing aerial song flights in the late evening and early dawn. The males, physically smaller than the females, play no role in raising young. Four eggs are usual, deposited in a scrape or hollow on the ground amid concealing vegetation. The precocious young are fed by their mother for the first week of life until their rapidly lengthening bills allow them to efficiently forage on their own. Although worms are the preferred staple, both adults and young take a variety of small invertebrates and seeds, even small frogs and fish have been recorded. Family groups typically break up about a month after hatching, in any given locality one brood each year being the norm.
Woodcocks winter across the southeastern United States from New Jersey to East Texas, where they feed upon native and introduced earthworms alike. In contrast to other shorebirds, they are low altitude migrants, flying at night just above treetop level. As is true of songbirds, losses may occur in collisions with man-made structures.
They are among the earliest migrants to arrive in the northern parts of their breeding range. Males begin to display on the wintering grounds as early as January, and possibly mate several times on their way north. Certainly it would seem possible for females to likewise breed more than once each year, but such has not yet been proven true.
Woodcocks are rare in winter in our local area, although these secretive birds are probably often overlooked. They do appear to be present in unusual numbers this year, doubtless due to the severe drought occurring further east. Despite their ongoing range expansion up north, overall numbers have been falling steadily in recent decades, perhaps due to habitat loss, both to development and to the continuing maturation of forests in formerly cleared areas.
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