The Worldwide Variation in Avian Clutch Size across Species and Space

Abstract

Traits such as clutch size vary markedly across species and environmental gradients but have usually been investigated from either a comparative or a geographic perspective, respectively. We analyzed the global variation in clutch size across 5,290 bird species, excluding brood parasites and pelagic species. We integrated intrinsic (morphological, behavioural), extrinsic (environmental), and phylogenetic effects in a combined model that predicts up to 68% of the interspecific variation in clutch size. We then applied the same species-level model to predict mean clutch size across 2,521 assemblages worldwide and found that it explains the observed eco-geographic pattern very well. Clutches are consistently largest in cavity nesters and in species occupying seasonal environments, highlighting the importance of offspring and adult mortality that is jointly expressed in intrinsic and extrinsic correlates. The findings offer a conceptual bridge between macroecology and comparative biology and provide a global and integrative understanding of the eco-geographic and cross-species variation in a core life-history trait.

Author Summary

Why do some bird species lay only one egg in their nest, and others ten? The clutch size of birds is one of the best-studied life-history traits of animals. Nevertheless, research has so far focused either on a comparative approach, relating clutch size to other biological traits of the species, such as body weight; or on a macroecological approach, testing how environmental factors, such as seasonality, influence clutch size. We used the most comprehensive dataset on clutch size ever compiled, including 5,290 species, and combined it with data on the biology and the environment of these species. This approach enabled us to merge comparative and macroecological methods and to test biological and environmental factors together in one analysis. With this approach, we are able to explain a major proportion of the global variation in clutch size and also to predict with high confidence the average clutch size of a bird assemblage on earth. For example, cavity nesters, such as woodpeckers, have larger clutches than open-nesting species; and species in seasonal environments, especially at northern latitudes, have larger clutches than tropical birds. The findings offer a bridge between macroecology and comparative biology, and provide a global and integrative understanding of a core life-history trait.

Abstract

Why do some bird species lay only one egg in their nest, and others ten? An analysis combining comparative and macroecological approaches across more than half of all bird species explains the global variation in this trait with high confidence.