Sexual differences in behaviour and morphology of Northern Gannets

Northern Gannets (Morus bassanus) are large, long-hved seabirds with negligible sexual size dimorphism. In common with other species where the sexes appear similar, very little is known about the differences in breeding behaviour of males and females. I used DNA-based sexing to investigate the accuracy of externally measured biometrics, plus morphological and behavioural observations for sexing breeding birds. I then measured how foraging trip durations, nest attendance patterns and levels of non-attendance by males and females changed during chick-rearing. All individuals were sexed correctly from nape biting behaviour, but other variables were less successful as a result of lower accuracy and/or because they could be recorded only relatively infrequently. Males spent a significantly higher proportion of time at the nest than females (53% and 42% respectively) and made shorter, more frequent foraging trips than females (mean = 17.3 hours S.D. ± 8.2, and mean = 21.0 hours S.D. ± 8.4 respectively). The frequency of food delivery by males increased with increasing chick age, but food delivery rates by females did not change as chicks grew. These data may indicate a greater contribution to food provisioning by males than females. The proportion of time that chicks were left unattended increased with chick age, while males were more likely to leave chicks unattended than females. This probably resulted from males making a trade-off between protecting the chick and providing food for the chick or foraging for themselves. The degree of equitability in foraging trips and nest attendance were not related to either chick hatching date or fledging success.