Factors Affecting Nest Site Location in Gila Woodpeckers

Abstract

We investigated the locations of Gila Woodpecker (Melanerpes uropygialis) nest sites in Organ Pipe Cactus National Monument, Arizona. A significantly greater proportion of saguaro cacti in arroyos had woodpecker holes than did those on hillsides, ridgetops or desert flats. Saguaros used for nest-hole excavation were taller and more likely to have branches than randomly chosen saguaros. Both active and inactive nest holes were nonrandomly oriented, with the mean vectors of orientation being in northwesterly directions. Variation in nest-hole orientation probably is not due to seasonal differences in excavations because the orientation of spring active (late March) nest holes was not significantly different from that of inactive holes. Some variation in nest-hole orientation is attributable to the structure of the saguaros. Desert birds are often thought to be affected primarily by thermoregulatory constraints in placing their nests (Ricklefs and Hainsworth 1969, Austin 1974). Recently, Inouye et al. (1981) considered the nonrandom orientation of Gila Woodpecker (Melanerpes uropygialis) nest holes in saguaro cacti (Cereus giganteus) to represent such an adaptation. In addition, they suggested that variability about the mean nest-hole orientation might reflect a thermal need for woodpeckers to use holes that are oriented differently at different times of the year or season. Old woodpecker excavations constitute a volume of valuable historical information that is unique and rarely available to animal ecologists. In this paper we use such information to test the hypotheses that Gila Woodpeckers use a random subset of saguaros for nesting (or roosting) purposes, and that the position and orientation of holes within those saguaros are random. Only the orientation of nest holes has received attention before this study (Inouye et al. 1981). We then compare the mean orientation of inactive holes with that of holes which were being actively used during early spring, to test the hypothesis that the mean orientation of active holes shifts seasonally. Finally, we re-evaluate the possibility that thermal constraints alone can adequately account for the nest site locations of this wood-