No abstract

1. 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 defences. 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.2. 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. 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 nesting habitat.3. Colony size was identified as the only significant predictor of both innate (natural antibody-mediated complement activation) and adaptive (immunoglobulin concentrations) immunological traits in the common terns, as more immunocompetent birds settled in larger experimental colonies. In contrast, we found no significant associations between colony size and genetic diversity of key pathogenrecognition receptors, toll-like receptors (TLRs) and the Major Histocompatibility Complex (MHC) or genome-wide heterozygosity.4. We conclude that settlement decisions may be flexible within individuals and, thus, are likely to be primarily determined by the current immunological status, rather than fixed immunogenetic traits. Our study sheds new light on the complex interface between immunity and sociality in animals.

The choice of colony size may have profound consequences for individual fitness in colonially breeding birds, but at the same time it may require certain behavioural adaptations. Here, we aimed to examine behavioural divergence of common terns Sterna hirundo nesting in colonies of different size. For this purpose, we promoted establishment of small (<35 pairs) and large (>100 pairs) tern colonies under uniform ecological and environmental conditions by providing attractive patches of nesting substrate (floating rafts) at a single site. We combined video recording and GPS-tracking to assess communal and individual defence initiation rate, intra-specific aggression rate, and foraging flight characteristics. We found that birds from larger colonies more frequently engaged in communal defence and they performed longer foraging flights, while terns from smaller colonies more frequently showed individual defence behaviours. Also, intra-specific aggression rate was higher in smaller colonies, but this effect was primarily attributed to a higher proportion of edge breeding pairs, which were more aggressive. Our results suggest that various colony sizes may be associated with different behavioural syndromes, which comprise of diverse personality traits, such as social responsiveness, social tolerance, or propensity for aggression. It remains to be tested whether these behavioural differences reflect processes of phenotypic sorting among colonies of different size or whether they are a result of behavioural plasticity under different social contexts.

Most behavioral traits are known to be weakly heritable, possibly due to their extreme complexity and flexibility. Despite this general pattern, within-species variation in avian colony size choice has been reported to have a strong additive genetic component, but we are aware of no attempts to assess the heritability of avian sociality at the finer spatial scale. Here, we used an animal model and parent–offspring regression to quantify additive genetic variance in social phenotype (local nesting density) in a nonpasserine waterbird, the common tern Sterna hirundo. For this purpose, we used a novel experimental framework, where variation in the social environment was generated by providing birds with artificial patches of attractive nesting substrate that markedly varied in size. During 2011–2019, we collected data on social preferences for either low or high nesting density in over 250 individuals, either kin (mostly parent–offspring relationships) or non-kin recorded breeding multiple times across years. All heritability estimates of local nesting density were low (&lt;0.10), irrespectively of fixed effects (sex and year) included in the models, data used in the modeling (all individuals vs. early recruits), or methodological approach (animal model vs. parent–offspring regression). We conclude that avian sociality, as measured at the local scale, may be much less heritable than colony size choice, as measured at the landscape level. Our study adds to the understanding of additive genetic variance in avian behavior, and it underlines a scale dependency in the heritability of behavioral traits.