Opposing selective pressures may act on the colony size in a waterbird species

Abstract

In theory, larger colony size should be favoured by lower per-capita predation rates, whereas smaller colony size should be favoured by reduced parasitism, social stress and competition for food. We conducted an experimental cross-fostering of young between colonies of different size to test whether differences in fitness had an environmental or genetic basis. We induced formation of one large (ca. 100 breeding pairs) and three small (30–40 pairs) Common Tern Sterna hirundo colonies by providing different size patches of nesting area (floating rafts). The larger colony had about 30 % higher reproductive success than the smaller colonies. However, offspring raised in the large colony were in poorer condition, which was indicated by the higher heterophil/lymphocyte ratios, lower hemoglobin concentrations in blood and slower growth rates, suggesting higher parasite loads or higher levels of social stress. By performing a cross-fostering experiment we confirmed that differences in chick condition were not inherited, but could be directly attributed to different rearing conditions experienced in the colonies of varying size. These results suggest that colony size in the Common Tern may be regulated by the opposing selective pressures (predation vs. parasitism/social stress).