Evidence of nest abandonment in Grass Wrens Cistothorus platensis suggest that non-breeding nests are not structures that differ in function from breeding nest

Abstract

Several hypotheses propose an adaptive function of non-breeding nests (female attraction, decoys for nest predators and safe places for roosting). Alternatively, non-breeding nests may be structures abandoned before completion. Grass Wrens Cistothorus platensis build two types of non-breeding nests: platform and dummy nests. Platforms are rudimentary accumulations of grasses whilst dummies are dome-shaped, similar in appearance to breeding nests. We assess nest ontogeny, structure, dimensions and placement of non-breeding and breeding nests to provide evidence of abandonment. We predict that non-breeding nests would lack several of the nest layers present in breeding nests but would not differ in nesting microhabitat. Platforms had an incomplete structural layer and lacked most of the functional layers present in the breeding nests. Dummy nests had a complete structural layer, but often lacked some of the layers (lining and nest cup) that were added during the later stages of building. Breeding and non-breeding nests did not differ in seven nesting microhabitat variables. These results are in line with the hypothesis that platforms and dummy nests are structures abandoned during the building process. Differences between non-breeding and breeding nests in both nest and entrance orientation suggest that nests with an NNE orientation were less likely to be abandoned. Hence, wrens may abandon nests if they perceive unsuitable ambient conditions at the nest site. However, other causes of desertion such as the encounter of a predator during building were not evaluated. The study of non-breeding nests may provide the opportunity to evaluate the costs and benefits of nest desertion as well as insights in how function can shape nest architecture and location.