Nest site selection by a secondary cavity-nesting species, the mountain bluebird
Abstract
As secondary cavity-nesters, mountain bluebirds are limited in their selection of nest sites by the choices of primary (excavating) cavity nesters. In the aspen forests of Colorado, red-naped sapsuckers are the primary excavators and several species depend on sapsucker nest cavities for their own nests. Our goal was to determine whether secondary cavity-nesters selected particular nest cavity characteristics from the array of available nests, or if their choice was random. We focused on mountain bluebirds because most previous studies used populations nesting in artificial nest boxes. In order to determine the characteristics of mountain bluebird nests we measured diameter at breast height (DBH), nest height, percent tree cover, nest opening orientation and distances to edge of aspens, nearest willow and nearest water. We compared these characteristics of mountain bluebird cavities to the same characteristics of cavities occupied by other secondary cavity nesters and we found very few differences. Bluebirds nest in trees with a slightly larger DBH and appear to nest farther from willows and water than other secondary cavity nesters. Bluebird cavity openings are also non-randomly oriented to the east-southeast; however, this was not significantly different from orientation of other cavities. All other analysis revealed no significant differences. We conclude that mountain bluebirds select larger-diameter trees than do other secondary cavity nesters. We also suggest that the strongest determining factor of mountain bluebirds could be a characteristic that we did not measure, namely distance to other mountain bluebird nesting pairs. We suggest further study on territory requirements of mountain bluebirds to test this hypothesis.
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