The Worldwide Variation in Avian Clutch Size across Species and Space

Walter Jetz,Cagan H Sekercioglu,Katrin Böhning-Gaese,Ben Sheldon

doi:10.1371/journal.pbio.0060303

Walter Jetz, Cagan H Sekercioglu + Show 2 more

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https://doi.org/10.1371/journal.pbio.0060303

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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.

Highlights

There is enormous variation in life-history among species and across regions, which ecologists have long sought to explain [1,2,3]
Lack [3,11] hypothesised that clutch size may be determined by food abundance during the breeding period, per se, and that northern species have large clutches because daylight periods during the breeding season are longer than those in the tropics
Classical life-history theory predicts that high seasonality in the temperate regions, causing high adult mortality, will lead to the evolution of high investment in current reproduction and large clutch sizes because the likelihood to survive until the breeding season is low [5]

Summary

Introduction

There is enormous variation in life-history among species and across regions, which ecologists have long sought to explain [1,2,3]. Ashmole [9] argued that high adult mortality in the temperate regions reduces population density, increases per-individual resource availability in the breeding season, and allows temperate birds to nourish large clutches [4,12] Like these suggested extrinsic drivers, clades and their intrinsic traits that may affect clutch size are not randomly distributed along environmental gradients or realms. We compiled information on clutch size and other intrinsic (body mass, migratory behavior, development mode, nest type, diet) and extrinsic attributes (latitude, temperature, precipitation, net primary productivity, seasonality, and realm) for a total of 5,290 species of landbirds This allows us to develop and test a first global model of clutch size that integrates existing viewpoints of lifehistory variation.

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Findings

Materials and Methods