I assessed nest-site characteristics of two cavity-excavating species, the Acorn Woodpecker (Melanerpes formicivorus) and Northern Flicker (Colaptes auratus), and two secondary cavity-nesting species, the Flammulated Owl (Otus flammeolus) and Western Bluebird (Sialia mexicana). These species were common in mixed coniferous-deciduous forests within my New Mexico study areas. I predicted that nests of the woodpecker species could be differentiated by cavity size and that the excavator species of nest cavities used by secondary cavity-nesters could be predicted with this model. Flickers excavated nest cavities that were larger and surrounded by denser forest than Acorn Woodpecker nests, and three characteristics of cavity size alone differentiated 84% of woodpecker nest sites. Acorn Woodpeckers nested in live Gambel oak (Quercus gambelii) most frequently and Northern Flickers in ponderosa pine (Pinus ponderosa) snags, and these differences may be attributed to the excavation ability of each species. The secondary cavity-nesters did not show a preference for trees of different condition (i.e., alive or dead). A multivariate regression model predicted that Flammulated Owls and Western Bluebirds should nest in cavities excavated by both woodpeckers, but owls showed a preference for Northern Flicker cavities and bluebirds for Acorn Woodpecker cavities. Owl nest cavities were larger and surrounded by denser forest than bluebird nest cavities, similar to the differences observed between woodpecker nest sites. I suggest that these differences can be partially attributed to body size as well as ecological similarities between the primary and secondary cavity-nesters. Western Bluebirds showed a preference for cavities facing south and east, and this may have been due to thermal advantages.
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Vol. 75 • No. 3