Abstract

Bird nests are highly variable structures that represent extended phenotypes, which may have a range of roles during avian reproduction, but all nests serve as the location for incubation of eggs. Nest characteristics should counter any adverse weather conditions that could potentially reduce hatchability or chick survival. Thermal properties of nests are relatively well studied, but the effects of moisture on nest insulation are largely unknown. Wet nests could reduce thermal insulation or deleteriously cool the eggs or nestlings. In this study, a novel method of simulated rainfall was applied to open-cup nests from nineteen different passerine species. The amount of water absorbed by the nest and the time it took to dry were recorded. To determine the factors affecting these characteristics, nests were deconstructed to provide data on materials used. Compared with smaller nests, larger nests absorbed more water and took longer to dry. The amount of water absorbed was a function of the proportion of grass, moss, roots and hair present in the nest. The time taken for the nest to dry completely was affected by the proportion of grass, moss, and hair in the nest. When tested individually, the materials all had different rates of water absorption and took different times to dry; moss absorbed the most water and took the longest time to dry. We concluded that simulated rainfall was a realistic approach to understanding the hydrological characteristics of nests. Structural considerations for nest construction may affect the hydrological properties of nests, but ex situ studies provide no information about the possible shelter provided to a nest by its immediate surroundings. Further research into nest function should consider the importance of exposure to water to nest construction and function.