Life history and the evolution of family living in birds

Abstract

The reason why some bird species live in family groups is an important question of evolutionary biology that remains unanswered. Families arise when young delay the onset of independent reproduction and remain with their parents beyond independence. Explanations for why individuals forgo independent reproduction have hitherto focused on dispersal constraints, such as the absence of high-quality breeding openings. However, while constraints successfully explain within-population dispersal decisions, they fail as an ultimate explanation for variation in family formation across species. Most family-living species are long-lived and recent life-history studies demonstrated that a delayed onset of reproduction can be adaptive in long-lived species. Hence, delayed dispersal and reproduction might be an adaptive life-history decision rather than 'the best of a bad job'. Here, we attempt to provide a predictive framework for the evolution of families by integrating life-history theory into family formation theory. We suggest that longevity favours a delayed onset of reproduction and gives parents the opportunity of a prolonged investment in offspring, an option which is not available for short-lived species. Yet, parents should only prolong their investment in offspring if this increases offspring survival and outweighs the fitness cost that parents incur, which is only possible under ecological conditions, such as a predictable access to resources. We therefore propose that both life-history and ecological factors play a role in determining the evolution of family living across species, yet we suggest different mechanisms than those proposed by previous models.

Figure 1

Proposed pathway leading to family formation in birds. The effects of life history and environmental factors on parental decisions are illustrated by the grey arrows, while the effects on offspring decisions are shown by the black arrows. The environmental factors are shown in boxes. The directions of the relationships between traits or factors are illustrated by ‘plus’ for positive correlations and ‘minus’ for negative correlations. Species with high survival tend to start breeding later in life and can invest more in offspring. These two factors combined can cause offspring to delay dispersal leading to the formation of families. Environmental factors such as good breeding conditions will promote dispersal. Resource availability allows parents to invest more in offspring without incurring strong costs. However, parents should only prolong investment in offspring if this increases offspring survival prospects substantially without compromising their own survival, and thus there should not be parental investment when offspring survival is too low or too high.