Nest Sites and Population Demographies of White-Breasted and Pigmy Nuthatches in Colorado

Abstract

of mountain regions in western North America. In north-central Colorado, these species breed principally in ponderosa pine (Pinus ponderosa) forests of the Transition Zone (Bailey and Niedrach 1965). Despite overall similarity in nesting habitat and breeding schedule, these species differ in nest site preferences and breeding population densities (Stallcup 1968). Both of these nuthatches use (a) natural cavities, (b) cavities bored out by other species, (c) cavities excavated by conspecifics, and (d) modify pre-existing cavities, or (e) excavate their own cavities (Bent 1948, Norris 1958). Very little is known about the historical background of cavities used for nesting; however, Pigmy Nuthatches excavate cavities more frequently than Whitebreasted Nuthatches (Bent 1948). But, even in the case of Pigmy Nuthatches, new cavities are excavated rarely (Norris 1958). Breeding populations of cavity-nesting birds often are limited by the number of suitable and available nest sites (von Haartman 1971, Hogstad 1975). Consequently, population densities of birds within an area, although demonstrating within-year fluctuations due to recruitment, death, and dispersion, exhibit little between-year variation in breeding population densities (Lack 1966). Stallcup (1968) showed that Pigmy Nuthatches have higher breeding densities than White-breasted Nuthatches within a ponderosa pine habitat; however, both species' breeding populations exhibited little annual variation in the number of breeding pairs. I undertook analysis of the nest sites used by these species to see whether nest site availability was a factor associated with the differing levels of breeding populations.