On the Mechanism of Mate Selection in Black-Headed Gulls

Processes leading to the establishment of social relationships are the main topic of the present study, which is based on observations of black-headed gulls kept in experimental groups in aviaries. Among these birds various kinds of social relationships were established, such as heterosexual monogamous pair-bonds, polygynous associations, and male-male relationships. Some of the factors which give rise to the formation of such relationships could be studied, because it was possible to watch all individuals during their whole life, and during all stages of their reproductive cycle. Quantitative data were collected on the influence of sex, age, reproductive state, physical condition, and various social factors on the incidence of social relationships. On the basis of these data it could be concluded that males and females play strongly differing roles. Unmated males advertize randomly. Unmated females direct their courtship to particular individuals, belonging to the category of advertizing unmated males, or of the active mated males. The final decision for pair-formation involves an acceptance by the approached male. Unsuccessful unmated males may adopt the female strategy, and thus establish homosexual social relationships. Mated males do not initiate courtship towards other females, but they may be courted by them. Mated females sometimes display social interest for other males. The main criteria for mate choice were advanced age, early nuptial moult, small body-size, and, in particular, social experience with one another. In contrast to our expectation, body weight and matching of nuptial moult and physical condition between partners were not important in our birds. The significance of the criteria age and moult may be directly related to fitness. The preference for small males had earlier (van Rhijn, 1985) been interpreted as a mechanism preventing polygyny, but additional data provide some inconsistencies with that idea. Finally, the significance of social experience with each other has been related with considerable investments of prospective mates before establishing a pair-bond, and with an increase of synchronization between mates as a result of long-lasting pair-bonds. Female-female pairing among some wild larids has been interpreted as a continuation of early bonds between nest companions.