Some avian species augment nest cups by building associated architectural structures that may mitigate predation, parasitism, and/or hatching failure. Because effective nest construction is integral to reproductive success, architectural structures associated with nests are predicted to provide functional benefits. Rock Wrens (Salpinctes obsoletus) stereotypically augment their soft cup nests with a pavement of stones, apparently incurring considerable energy costs. We quantified Rock Wren stone use and measured how stones occlude nest cavities. We examined whether Rock Wrens adjust individual stone-carrying effort in response to nest cavity opening size and tested 3 hypotheses about the benefits of cavity occlusion: (1) stones ameliorate temperature fluctuations and improve nest thermoregulation; (2) stones improve nest microclimates by keeping them dry; and (3) stones have the potential to reduce nest predation by alerting incubating females when predators approach. We found that individual nest pavements contained up to 1.4 kg of stones, which varied in size but were relatively uniform in thickness. Stone pavements decreased nest cavity openings by a mean of 34%, with larger openings containing significantly more stones. Presence of stones did not influence temperature in unoccupied nest cavities but did significantly decrease water infiltration into the nests during simulated rainfall. Presence of stones also changed the sound of a simulated predator approach, supporting the idea that stone patios could serve as an alarm function for vulnerable incubating females. Our data indicate that Rock Wrens adjust the amount of stones used in nests according to cavity characteristics to obtain multiple benefits. Results confirm that nest site modification can be an adaptive behavior and provide evidence that birds facultatively modify nesting environments.
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