Microclimate of Natural Cavity Nests and Its Implications for a Threatened Secondary-Cavity-Nesting Passerine of New Zealand, the South Island Saddleback

. Secondary-cavity-nesting birds occur widely throughout the world, but little information is available on the benefits of the nest's microclimate for such species, particularly for those using natural cavities. We investigated the influences of microclimate on a threatened secondary-cavity-nesting passerine, the South Island Saddleback (Philesturnus carunculatus carunculatus). Our aims were to determine whether (1) saddlebacks select tree cavities with microclimates less variable than those of other tree cavities in their surrounding territory, (2) whether structural aspects of tree cavities translate into certain microclimate characteristics, and (3) if less frequently used sites not in tree cavities (e.g., cavities in banks or in vegetation) have thermal properties similar to those of tree-cavity nests. We found that the saddleback's tree-cavity nests were more stable in temperature, more insulated against cold, and did not change temperature as rapidly as the ambient air or unused tree cavities. Regression analysis showed that of structural characteristics of tree cavities examined, only one, entrance width, was significantly associated with an aspect of microclimate (minimum temperature). Additionally, we found that regardless of cavity type the thermal properties of saddleback nest cavities were similar. These results indicate that saddlebacks likely select nest cavities with less variable thermal properties that are potentially beneficial, and future studies experimentally manipulating the variability of microclimate may be fruitful in determining the effect of microclimate on reproductive success. Nevertheless, this study is one of the first to demonstrate microclimate as a factor determining selection of natural nest cavities over available unused cavities.