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

Low heritability of social phenotypes in a nonpasserine waterbird

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 (<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.

Population density mediates induced immune response, but not physiological condition in a well-adapted urban bird

Thriving under high population density is considered a major feature of urban exploiter species. Nevertheless, population density appears to be a surprisingly overlooked factor in urban ecology studies. High population numbers observed in urban species might promote pathogen transmission and negatively affect health or condition, thus requiring investments in immunocompetence. The feral pigeon Columba livia domestica is an example of a successful city-dweller, found in great abundance in large cities across the globe. We investigated the effects of population density on induced immune response (phytohaemagglutinin skin test) and body condition (blood haemoglobin concentration and size-corrected body mass) in 120 feral pigeons, captured along population density gradient in Łódź (central Poland). We found that stronger immune response was associated with higher population density, but was not related to physiological condition and physiological stress (heterophil/lymphocyte ratio). Moreover, condition indices were not associated with population density. However, since pigeon population density was highly correlated with the level of habitat urbanization, we cannot exclude that any density-dependent effects may be mediated by habitat variation. Our results indicate that urban environment, via population density, might exert different selective pressures on immunocompetence and body condition in this successful urban exploiter.

Immunocompetent birds choose larger breeding colonies

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. 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. 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 pathogen-recognition receptors, toll-like receptors (TLRs) and the Major Histocompatibility Complex (MHC) or genome-wide heterozygosity. 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.

Density-dependence of nestling immune function and physiological condition in semi-precocial colonial bird: a cross-fostering experiment

BACKGROUND

Nesting in large aggregations provides several important advantages for colonially breeding birds. However, it also imposes certain costs, associated with facilitated pathogen transmission and social stress. The cost-benefit ratio is not similar for all the birds in a colony and it might be mediated by nest density. To investigate the influence of nest density on cell-mediated immune function and on physiological condition of nestlings, we arranged a cross-fostering experiment in three breeding colonies of black-headed gulls Chroicocephalus ridibundus. First, we exchanged eggs between plots of high and low nest density. Afterwards, we performed phytohaemagglutinin (PHA) skin test and we measured blood haemoglobin concentration in nearly 350 nestlings from experimental (exchanged) and control (non-exchanged) groups.

RESULTS

We found that PHA response was lowest in high nest density control group, indicating that depressed immune function of offspring, likely caused by social stress, can be considered as a cost of colonial breeding. Contrastingly, body condition of nestlings was the poorest in low density control group.

CONCLUSION

Nestlings hatched and raised in high nest density plots did not have higher blood haemoglobin concentration in comparison to other study groups. Furthermore, they were affected with depressed cell mediated immune function, which is possibly driven by combined maternal (corticosteroid hormones deposited in yolk) and environmental (elevated social stress) effects. These results indicate that breeders from high nest densities do not benefit by rising offspring in better quality, in terms of immune function and body condition, although, in the light of previous studies, high nest densities are occupied by birds of higher individual quality, than low density areas. Our study provides a novel insight into the mechanisms of density-dependence that govern fitness of colonially nesting birds.

Colony size as a predictor of breeding behaviour in a common waterbird

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.

Colony size affects nestling immune function: a cross-fostering experiment in a colonial waterbird

Elevated transmission rate of pathogens and parasites is considered one of the major costs of sociality in birds. However, greater risk of infection in colonial birds might be compensated by specific immune adaptations. Here, we predicted that nestlings raised in larger colonies should invest more in their immune function. To test this hypothesis, we manipulated colony size and conduced cross-fostering experiment in a colonial waterbird, the common tern Sterna hirundo. Establishment of different size colonies under uniform environmental conditions was induced by providing large and small patches of attractive nesting area for terns (floating rafts). Then, pairs of clutches were swapped between large and small tern colonies, and skin-swelling response to phytohaemagglutinin (PHA) was assessed for nestlings from experimental and control broods. Contrary to our expectations, we found a negative effect of foster colony size on nestling PHA response (nestlings raised in the larger colony had lower PHA response). In addition, nestling PHA response correlated negatively with heterophil/lymphocyte ratio used as a measure of physiological stress. This suggested that low PHA response of nestlings raised in the larger colony could be mediated by an elevated level of social stress. We suggest that depression of immune function via social stress may constitute a strong selective pressure against large colony size in the common tern, and possibly in other colonial species. We also recommend that this largely overlooked cost of sociality should be considered in the further studies on the evolution and ecology of avian coloniality.

The use of haemoglobin concentrations to assess physiological condition in birds: a review

Total blood haemoglobin concentration is increasingly being used to assess physiological condition in wild birds, although it has not been explicitly recognized how reliably this parameter reflects different components of individual quality. Thus, I reviewed over 120 published studies linking variation in haemoglobin concentrations to different measures of condition and other phenotypic or ecological traits. In most of the studied avian species, haemoglobin concentrations were positively correlated with other commonly used indices of condition, such as body mass and fat loads, as well as with quality of the diet. Also, chick haemoglobin concentrations reliably reflected the intensity of nest infestation by parasitic arthropods, and haemoglobin was suggested to reflect parasitism by haematophagous ectoparasites much more precisely than haematocrit. There was also some evidence for the negative effect of helminths on haemoglobin levels in adult birds. Finally, haemoglobin concentrations were found to correlate with such fitness-related traits as timing of arrival at breeding grounds, timing of breeding, egg size, developmental stability and habitat quality, although these relationships were not always consistent between species. In consequence, I recommend the total blood haemoglobin concentration as a relatively robust indicator of physiological condition in birds, although this parameter is also strongly affected by age, season and the process of moult. Thus, researchers are advised to control fully for these confounding effects while using haemoglobin concentrations as a proxy of physiological condition in both experimental and field studies on birds.