Abstract
BackgroundNesting 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.ResultsWe 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.ConclusionNestlings 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.
Highlights
Nesting in large aggregations provides several important advantages for colonially breeding birds
We aimed to address the influence of breeding pair density on chick immunocompetence and physiological condition in another colonial bird species, black-headed gull Chroicocephalus ridibundus
Our results showed that nestlings from high density control group (HH) had significantly lower PHA response (Fig. 1, Table 2) than birds from LL and low to high density (LH) groups and nearly significantly lower than birds from HL group, (P = 0.052, Table 2)
Summary
Nesting in large aggregations provides several important advantages for colonially breeding birds It imposes certain costs, associated with facilitated pathogen transmission and social stress. Individuals of lower quality are often relegated to smaller colonies, peripheral areas and lower nest densities, deprived of basic advantages associated with social breeding, and freed from its major costs. For these individuals predator threat is bigger, but the social stress and disease transmissions are diminished. The cross-fostering experiment performed in common tern colonies of different size, but under uniform nesting densities, revealed diminished immune response in tern chicks fostered in a larger colony, demonstrating costs of social stress imposed on immunocompetence [15]
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