Site and mate fidelity of Shags Phalacrocorax aristotelis at two British colonies.
The mate retention and site fidelity of marked Shags Phalacrocorax aristoteIis was studied on the Isle of May, southeast Scotland, from 1981 to 1983 and on the Farne Islands, northeast England, from 1963 to 1964.
Over 3 years, 76% of 3038 nest sites used by Shags breeding on the Isle of May were re-occupied in the following year. Movements of 461 marked individuals showed that ownership had changed on a third of the re-occupied sites; Shags that changed site moved preferentially to sites that had been occupied the year before. The proportion of first-time breeders aged 2 years was five times higher on sites not previously occupied than on ones used the year before.
The behaviour of Shags at the two colonies was similar. On average, 56% of surviving males nested on the same site in consecutive years, although 2-year-olds were four times less likely to remain than those over 6 years old; site fidelity was higher in males than in females. Shags that changed site moved little, with 87% of birds returning to within 16 m of their previous nest site.
Half of 306 marked pairs bred together the following year; non-return of the partner accounted for 46% of the separations. The divorce rate increased with distance between the nest sites occupied by a male in consecutive years, from 17% for males that did not move to 75% for males that moved over 8 m. This explained the much higher divorce rate of first-time breeders compared with older ones.
Only 6% of females bred on their former site in the absence of their former mate compared with 83% when he returned, while 48% of females whose male had moved joined him on his new site. Thus the female's attachment was to the male rather than to the site, and renewal of the pair-bond depended on the female's ability to locate the male after his return, which was a function of how far he had moved.
Nearly twice as many males moved after breeding failure than after success, and male site fidelity seemed to result from competition for sites as assets influencing reproductive success.