Abstract
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.
Highlights
Coloniality occurs in different groups of vertebrates including fish, reptiles, and mammals, but it is most widespread in birds, as approximately 13% of all avian species are classified as colonial breeders (Lack 1968)
We found no effect of original colony size on the nestling skin-swelling response to PHA (Table 1). Both full and reduced models indicated a significant effect of foster colony size on nestling immune function (Table 1; Fig. 1), as offspring raised in smaller colonies showed stronger PHA response (1.62 ± 0.05 mm vs. 1.44 ± 0.05 mm for small and large foster colonies, respectively)
Our cross-fostering experiment in the common tern showed that nestling immune function was associated with foster colony size
Summary
Coloniality occurs in different groups of vertebrates including fish, reptiles, and mammals, but it is most widespread in birds, as approximately 13% of all avian species are classified as colonial breeders (Lack 1968). Despite extensive research on the evolution and function of coloniality (Brown and Brown 2001), the relative importance of ecological and environmental factors that produce inter- and intra-specific. It might be expected that greater risk of infection from a greater diversity of pathogens in colonial species should be compensated by specific adaptations of their immune system. It has been shown that highly social species of birds develop larger immune defense organs (bursa of Fabricius and spleen) than solitary species
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
