Facultative Nest Modification by Rock Wrens ( Salpinctes obsoletus )

Abstract

Dynamic nest-construction behaviours suggest that nest structures can be facultatively adjusted in response to environmental conditions. This is particularly true when nest builders create environmental structures that are supplemental to the nest itself, because these structures can be adapted to nest site conditions. Rock Wrens (Salpinctes obsoletus) mechanically alter their nest environments by amending rock-based cavities with stones, which are placed below their soft nest cups and often occlude cavity entrances, and extending out of nest cavities. We measured stones and cavity dimensions in 68 Rock Wren nests to quantify stone form and functional variability in stone use. Nests in our study contained stones with a consistent, flat shape, suggesting that Rock Wrens were selective when choosing stones for use around nests. We found significant positive correlations between the size of the nest cavity entrance, the amount of stones placed in nests, and the occlusion rates of the nest cavity entrances. Our data suggest that Rock Wrens place stones around their nests using selective, goal-directed behaviour with the objective of approaching an optimal cavity entrance size. Stone use at nests reflects high behavioural flexibility, and can be considered tool use during nest construction.