Immunocompetent birds choose larger breeding colonies

The optimal size of social groups may vary between individuals, depending on their phenotypic traits, such as dominance status, age, or personality. Larger social groups often enhance transmission rates of pathogens and should be avoided by individuals with poor immune defenses. In contrast, more immunocompetent individuals are expected to take advantage of larger group sizes (e.g. better protection, information transfer) with smaller extra costs from pathogen or parasite pressure.

Here, we hypothesized that immunocompetence may be a key determinant of group size choice and tested this hypothesis in a colonial waterbird, the common tern Sterna hirundo. For this purpose, we used a unique experimental framework, where formation of breeding colonies of different sizes was induced under uniform environmental conditions. For this purpose, different-size patches of attractive nesting substrate (artificial floating rafts) were provided at a single site with limited availability of natural n...