We study the evolution of social behaviour and life history patterns in social animals, to discoveri why and how animals cooperate or compete with each other. Our research focuses on behaviour as this is the level at which organisms interact most directly with their physical and social environment. We integrate empirical and theoretical studies to obtain deeper knowledge of the evolution of adaptive behavioural responses. We focus on long-term studies of wild birds in naturally varying environments; these are complemented with experimental studies at the individual and local population levels. We established one of the best known model systems in evolutionary ecology: the Seychelles warbler. Warblers normally produce one chick per nest and can be cooperative breeders, in which more than two individuals are engaged in raising the single offspring. Our main research focuses on two groups of questions.

What drives the evolution of group living and cooperative breeding?

Offspring often delay dispersal and remain in their natal territory. My work provided confirmation of two much-debated hypotheses on the ecological factors influencing delayed dispersal. I discovered that the main cause of such delay is the degree of habitat saturation. However, the amount of insect prey available in a territory is also an important factor. For a young bird, the breeding success and survival benefits of remaining and helping in good territories (high food abundance) outweigh the benefits of independent breeding in poor areas, and offspring from good territories rarely disperse to breed in poor areas. We found that most helpers are females, usually daughters from previous broods, and that it is the males who typical disperse from their natal territory. Helpers increase their fitness indirectly by preferentially feeding more closely related offspring, and directly through gaining breeding experience and parentage within their own group. For breeding pairs living in high-quality territories, having helpers increases their reproductive success and survival, but in low-quality territories helpers compete with breeders for food resources.

What drives the evolution of sex allocation?

We showed that mothers in high-quality territories produce mainly daughters (the helping sex), while in low-quality territories they produce mainly sons (the dispersing sex). This skew in the sex ratio is striking, because birds were thought to be unable to skew sex ratios (their sex is determined by chromosomes). By producing more daughters in high-quality territories, mothers gain future helpers, lightening their own provision load, increasing total feed to nestlings, and improving their survival, reproductive success, and longevity. In contrast, mothers in low-quality territories produce dispersing offspring and so avoid competition over food resources that would lower reproductive success. In addition, daughters born in high-quality territories have higher survival and reproductive success than sons, while the reverse is true for low-quality territories. My research has stimulated wider interest in how vertebrates manipulate offspring sex ratio

As food availability and its spatio-temporal variation have changed, so the selection pressures on individuals have, and have forced them to adopt other strategies. We now need to investigate how unpredictable environmental changes affect how animals cooperate or compete with one another, and the implications of environmentally induced sociality on per capita and long-term fitness.