Parental care is one of the most variable social behaviors and it is an excellent model system to understand cooperation between unrelated individuals. Three major hypotheses have been proposed to explain the extent of parental cooperation: sexual selection, social environment, and environmental harshness. Using the most comprehensive dataset on parental care that includes 659 bird species from 113 families covering both uniparental and biparental taxa, we show that the degree of parental cooperation is associated with both sexual selection and social environment. Consistent with recent theoretical models parental cooperation decreases with the intensity of sexual selection and with skewed adult sex ratios. These effects are additive and robust to the influence of life-history variables. However, parental cooperation is unrelated to environmental factors (measured at the scale of whole species ranges) as indicated by a lack of consistent relationship with ambient temperature, rainfall or their fluctuations within and between years. These results highlight the significance of social effects for parental cooperation and suggest that several parental strategies may coexist in a given set of ambient environment.

Department of Animal and Plant Sciences University of Sheffield Sheffield S10 2TN United Kingdom;

Department of Biology and Biochemistry University of Bath Bath BA2 7AY United Kingdom; State Key Laboratory of Biocontrol and College of Ecology and Evolution Sun Yat sen University Guangzhou 5102275 China

Department of Limnology University of Pannonia H 8201 Veszprém Hungary;

Department of Zoology and Laboratory of Ornithology Palacky University 77146 Olomouc Czech Republic;

MTA DE Lendület Behavioural Ecology Research Group Department of Evolutionary Zoology University of Debrecen 4032 Debrecen Hungary;

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Dryad
10.5061/dryad.02JK0