Orientation of American Kestrel Nest Cavities and Nest Trees

Abstract

Balgooyen (1976), in his Sierra Nevada study, showed that nest cavities of American Kestrels (Falco sparverius) face east significantly more often than expected, and that nest trees are most often located on east-facing slopes. He based his results on comparisons of observed frequency distributions to a uniform distribution. But American Kestrels, typical of most secondary cavity-nesting birds, use abandoned nest cavities excavated by woodpeckers. It is possible, therefore, that the nonrandom orientations of kestrel nest cavities and slope exposure reflect selection by the original nest excavators rather than by the kestrels. Some studies have reported nonrandom orientations of woodpecker nest cavities (Lawrence 1967, Conner 1975, Inouye 1976, Korol and Hutto 1984). If most available cavities or slopes faced east, the apparent preference by kestrels could be random selection of available cavities or trees. To test this possibility, I compared Balgooyen's (1976) data on nest orientation to an independent sample of 105 nests of the two species whose nests kestrels most often use in California-Northern Flicker (Colaptes auratus) and Lewis' Woodpecker (Melanerpes lewis; Raphael and White 1984). I collected data from 1975 to 1978 on the same study areas reported by Balgooyen (1976). My null hypotheses were that: (1) no difference existed in orientation of kestrel nest cavities compared to that of available cavities, and (2) slope exposures were equal for kestrel nest trees and available trees. I tested the hypotheses using Watson's U2 test (Batschelet 1965:35). Both hypotheses were rejected. Nest cavity orientation differed significantly from expected (U2105,58 = 0.307, P < 0.005), and slope exposures also differed from expected (U293,58 = 0.490, P < 0.005; Table 1). Available cavities were generally oriented in a northerly direction (mean azimuth = 14*; Table 1), whereas most kestrel cavities faced east-northeastward (mean angle = 59*). Angular dispersions, which measure the relative concentration of points around the mean, were similar among nests and available trees for both cavity orientation and slope exposure (Table 1). Examination of differences between percentages of kestrel and woodpecker nests for each of the eight directions showed the greatest deviation for east-facing cavities and slopes (Table 1), supporting Balgooyen's original observations. Balgooyen (1976) speculated that nest cavities facing eastward might offer thermoregulatory advantages because of warmth of the morning sun and protection from storms and hot afternoon temperatures. These considerations should also apply to the woodpeckers that excavated the nests. If so, most woodpecker nests should have been excavated with east exposures. It is possible that woodpe kers excavated cavities at the place around t e tru k where decay conditions were best for nest excavation. In the present study area, however, both Lewis' Woodpecke s and Northern Flickers excavated nests in trees that were decayed throughout (Raphael and White 1984). Therefore, it is unlikely that their nests were oriented in relation to variable decay characteristics of the trees. While these results do not rule out the possibility that kestrels choose cavities to maximize thermal advantages, further study would be necessary before accepting such an explanation. The apparently nonrandom selection of east-facing cavities by kestrels nevertheless cannot be attributed to cavity availability.