Enclosed nests of birds: to avoid predation or to regulate microclimate?

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Stephanie Y. Chia, Yu Tsai-Chen and Mao-Ning Tuanmu, Biodiversity Research Center, Academia Sinica, Taipei, Taiwan

Background/Question/Methods
Nests are where birds incubate their eggs and raise the young. The characteristics of nests reflect various strategies that birds use to maximize their reproductive success in diverse biotic and abiotic environments. Especially, the evolution of enclosed nest structure has been explained by two main hypotheses. One believes that enclosed nests serve mainly to reduce the risk of predation, while the other suggests that the structure functions mainly to regulate the microclimate conditions in the nest. Previous studies have been carried out in different regions of the world and on different groups of avian species, but led to contradictory results supporting one or the other hypothesis. The objective of this study is thus to revisit the debate on whether enclosed nests evolved to reduce predation risk or to regulate microclimate at the global scale, encompassing the vast majority of the world’s avian species. We collected the nest structure information for more than 8,000 species from published text, images and videos, and coupled it with each species’ distribution range to construct the global distribution of nest structure. We then examined the spatial pattern of the proportion of species using enclosed nests across the globe to test the two hypotheses.
Results/Conclusions
Our results showed that 41% of the avian species only use enclosed nests, including dome-shaped nests, self-excavated cavities (primary cavities) and existing cavities that are naturally formed or excavated by other animals (secondary cavities), while 55% of the species use only open nests, including cup, platform and scrape nests. Latitudinal distribution of the nest structure revealed that the proportion of species using enclosed nests tended to increase towards the equator, likely supporting the predation risk hypothesis. However, the geographic distribution of each enclosed nest type exhibits different patterns, with primary cavities distributed across the rainforests, secondary cavities more common in the temperate regions, and domed nests particularly common in the Afrotropical region and Southeast Asia. The divergent patterns suggest that climates, nest niche availability or the avian evolutionary history may also play a role in this process, which warrants further studies. By including >80% of extant avian species, this comprehensive examination addressed a long-standing debate on the ecological drivers of the evolution of enclosed nest structure. Findings of this study may also improve our understanding of the potential effects of the shifting climate and predation regimes on different avian species under global change.