Evolution of cooperative breeding
Offspring often delay dispersal and remain in their natal territory. We 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. Our work therefore confirmed two much-debated alternative hypotheses on the ecological factors influencing delayed dispersal and led to a new synthetic view.
We showed that in cooperative breeding systems breeders reduce their parental care and increase in health (lower oxidative stress, higher body mass, less susceptible for diseases) when assisted by helpers, resulting in, delaying breeder senescence and enhances life expectancy. We also showed that helping deteriorates health and accelerates senescence in helpers, thereby reducing life expectancy. We found that within-individuals extra-pair parentage of both males and females declines in older age, and that males are cuckolded less in older age, furthering understanding of the evolution of infidelity. We also demonstrated intergenerational effects of parental age on offspring fitness, with mothers having daughters with reduced lifespan and lifetime reproductive success as the mother became older. These intergenerational effects can therefore have important effects on population and evolutionary dynamics.
Evolution of sex allocation
We showed that mothers in high-quality territories mainly produce daughters (the helping sex), while in low-quality territories they mainly produce sons (the dispersing sex). 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. This skew in offspring sex ratio is striking, because birds were previously thought not to be able to adjust the sex ratio at birth (as in birds sex is determined by chromosomes). We did not only find out that Seychelles warblers can adjust the sex ratio of their offspring, but in a series of meticulous studies, we also unravelled that they do so in a highly adaptive manner. Our research (which includes evolution of sex allocation on other systems) has stimulated wider interest in how vertebrates manipulate offspring sex ratios.
Animal personalities are behavioural traits that vary consistently (repeatably) between-individuals. Exploration of a novel environment and of a novel object are repeatable in the Seychelles warbler, with the former also being heritable. Exploration of a novel environment has a functional basis, being linked to dispersal behavior, with more exploratory males more likely to delay natal dispersal and more exploratory females more likely to disperse longer distances. Exploratory tendency was however not linked to fitness components such as longevity, age at first breeding, reproductive lifespan, lifetime reproductive success or extra-pair paternity. Selection therefore maintains variation in these behavioural phenotypes.
In the 1960s, only ca 29 Seychelles warbler remained in one hectare of mangroves on Cousin Island, with populations on other islands wiped-out due to habitat loss and invasive predators. Given the vulnerability of a species confined to a single small population, and that warblers virtually never move to other islands themselves, birds were successfully translocated from Cousin to four other suitable islands (led by Nature Seychelles). The world population of Seychelles warblers is now estimated at 3000 adult birds across five islands, and the conservation status of the Seychelles warblers has been reduced from endangered to vulnerable (IUCN 2013).