Immunocompetence and colony size choice in a colonial waterbird, common tern Sterna hirundo

Background/Question/Methods

Individuals are likely to choose social groups of different size according to their phenotypic traits, such as age, competitive ability, or personality. However, larger social groups can increase a risk of infection and should be avoided by individuals with poor immune defenses. In contrast, individuals with high level of immunocompetence are expected to prefer larger groups, as they can overcome the costs caused by pathogen pressure and get benefits associated with large aggregations (communal mobbing of predators or information transfer). We tested the hypothesis on the immunocompetence-based colony size choice in a colonial waterbird, the common tern Sterna hirundo. For this purpose, we experimentally induced formation of breeding aggregation of different size under uniform environmental conditions by providing small and large floating rafts at a site with limited availability of natural nesting habitat. In 2014-2019 we captured 140 adult individuals breeding in small and large colonies (all captured during incubation). To evaluate immunocompetence of captured terns, we assessed genetic and non-genetic components of both innate and adaptive immune response. The genetic component of immune response was assessed by genotyping pathogen-recognition genes of the innate (toll-like receptors, TLRs) and adaptive (the major histocompatibility complex, MHC) immune system. In these analyses we also controlled for the level of genome-wide heterozygosity, as estimated based on a panel of several thousand SNPs genotyped with ddRAD sequencing approach. The non-genetic component of immune response was assessed using two approaches: natural antibody mediated complement activation response (NAb, innate immunity) and IgY immunoglobulin level (adaptive immunity).

Results/Conclusions

Our results showed that birds which bred in large colonies had stronger innate (NAb response) and adaptive (IgY) immunocompetence when compared with birds nesting in small colonies. Colony size was identified as the only significant predictor of both innate and adaptive non-genetic immune traits. In contrast, we found no significant associations between colony size and genetic diversity of key pathogen-recognition receptors, TLRs and the MHC, or genome-wide heterozygosity. We conclude that settlement decisions may be flexible within individuals, as they are likely to be primarily determined by the current immune status, rather than fixed immunogenetic traits.