Parental age and offspring ectoparasite load in European Shags Stictocarboaristotelis

While age related improvements in breeding success have been reported in ma ny avian species, the underlying mechanisms are not well understood. Ectopa rasite load can have a significant deleterious effect on many aspects of ho st fitness. If younger breeders carry high parasite loads, and this has a d etrimental effect on their chicks, this could contribute to the poorer bree ding success of younger pairs. We measured loads of the conspicuous ectopar asitic louse Eidemanniella pellucida (Phthiraptera: Amblycera) on chicks re ared by young and older breeding Shags Stictocarbo aristotelis. Since young Shags breed relatively late in the season, and lay smaller eggs, we used a cross-fostering procedure to control for environmental and egg quality eff ects on chick louse loads. We found that, independent of breeding period an d whether the chicks hatched from eggs laid by young or older birds, broods raised by young pairs carried a significantly higher louse load than those raised by older pairs. In our data set, we could not detect an effect of l ouse load on offspring growth and survival when controlling for parental ag e in the statistical analysis. However, to examine this fully, an experimen tal manipulation would be required to break the otherwise close relationshi p between parental age and louse numbers.