Attributes of mountain bluebird cavity nests

Abstract

There are two types of cavity nesting birds: excavators and non-excavators. In the aspen (Populus tremuloides) woodlands of Colorado, the red-naped sapsucker (Sphyrapicus nuchalis) is the predominant primary cavity nester. Sapsuckers excavate new holes each year in trees infected with the heartwood rot fungus (Phellinus tremulae). A common secondary cavity-nester is the mountain bluebird. We tested if characteristics of bluebird nests were non-random in comparison to other secondary cavities. The characteristics considered were DBH, height of nest, cavity opening orientation, percent tree cover, and distance to nearest water, willows and edge. We found that mountain bluebirds inhabit trees with significantly larger diameters than other trees containing secondary cavity nesters. We suggest this is for thermoregulation purposes, because larger trees have a higher average temperature within the cavity. The cavity opening orientation was significantly non-random facing east-southeast, although this was not significantly different from other secondary cavity nesters. This may also be for thermoregulation, as this is the sunny side of the tree. Mountain bluebird cavities were slightly further from willows and water. A reason for that may be that more aggressive secondary cavity nesters may out-compete bluebirds from higher quality sites that are closer to water and willows. All other factors considered were not significantly different from other secondary cavity-nests, but based on observation we suggest that bluebirds may have large territories. They may also be generalists when it comes to specific nest site characteristics, but the distance to nearest conspecific should be considered in future studies.