The influence of adult age on territorial attendance by breeding Great Skuas Catharacta skua: an experimental study

Territorial or nest attendance by adult breeding seabirds is often essentia l to prevent egg or chick mortality. As a result, parent birds, when not en gaged in foraging trips, should stay on their territories whenever there is a risk from leaving their eggs and chicks unattended. Differences in terri torial attendance and time spent foraging by birds of different ages have b een interpreted as resulting from differences in adult foraging ability or in motivation to invest in the current reproductive attempt. However, most studies have failed to take into account seasonal variations in food availa bility, which could affect these patterns, because timing of breeding is of ten dependent on adult age. We designed an experiment to investigate the le vels of territorial attendance displayed by Gnat Skuas of known age facing the same environmental conditions and with similar food requirements. This was achieved by manipulating clutches so that all study pairs hatched their chicks at the same time. Pairs with one chick had a higher attendance (mea n number of adults per territory) than pairs with two chicks. This suggests that the need to increase the time foraging in response to the nutritional demands of a larger brood overrides any effect due to differential motivat ions in the amount of parental effort to be invested in brood protection. P airs with older males had a higher attendance than pairs with younger males . About 61% of the estimated foraging time of the pairs fell on the males. Attendance and adult age were not correlated with chick body condition. We conclude that pairs with two chicks that had high attendance achieved this by bring more efficient foragers, and not by provisioning their chicks less well. Therefore, our results suggest that older males are better foragers than younger birds. Female age did not correlate with attendance levels. Ho wever, pairs with older females were less likely to leave their chicks unat tended, which suggests that those pairs had a better co-ordination of their foraging and guarding activities. Breeding success in Great Skuas improves with adult age. Thus, our study identifies a mechanism by which older bird s can achieve better performance than younger individuals. Old birds are le ss likely to leave their chicks exposed to predation by conspecifics, which is the main cause of pre-hedging mortality in this species.