Can parental investment reduce social altruistic behaviour in Yellow-legged Gulls Larus michahellis?

This study aims to test if there are differences in nest defence behaviour in single and in groups of Yellow-legged Gulls Larus michahellis during two stages of the breeding season: incubation and early chick-rearing period. When a human intruder wearing a mask approached and stood still next to the target nest during incubation, the gulls took part in passive mobbing and helped the ‘attacked’ gull, thereby showing altruistic behaviour. In contrast, during the early chick-rearing period, decreased altruistic behaviour was observed: the individuals that took part in the passive mobbing tended to remain on their own nests, in order to look after their own chicks. In this stage, a reduction of the size of passive mobbing was also noted. Furthermore, gulls from disturbed nests increased the intensity of nest defence by increasing the number of dives directed at the heads of the human intruders. Again, those gulls whose nests were directly affected by an approaching human intruder left their nests quickly to begin an aerial defence, encouraging the chicks to leave the nest and hide among rocks and shrubs. The adult gulls came back to their nests only after the danger had ceased and the chicks had come back to their nests, in agreement with the Parental Investment Theory.