Maternal antibody transmission and breeding densities in the Black-headed Gull <i>Larus ridibundus</i>

1. The offspring of avian species, especially those of colonial breeders, are exposed to a number of pathogens immediately after birth. The chick's immune system is, at that early stage still immature and inefficient. As a consequence, diseases can have a strong impact on chick survival.2. The ability of mothers to transmit passive immunity in terms of antibodies of their own acquired immunity to their chicks is probably an essential pathway to enhance the chick survival. Since the production of antibodies is costly, females are expected to adjust the transmission of passive immunity to the local disease environment.3. We found that in Black-headed Gulls (Larus ridibundus L.) yolk antibody concentrations are positively correlated with local breeding density. This transmission pattern is likely to be adaptive, as the aggregation of birds will enhance the local presence of pathogens.4. When birds were forced to re-settle after the colony had been flooded (10-20% of the original number of breeding pairs re-settled), this relationship was no longer present. The lack of such a relationship may be explained by the fact that females may retain certain levels of antibodies as a consequence of infections that occurred during the first breeding attempt at a different breeding density.5. Within clutches, maternal antibodies decreased with laying sequence, in particular in eggs hatching male chicks. This transmission pattern may contribute to the observed mortality pattern found in gull species.