International audienceSocial monogamy is common among birds, while genetic monogamy is supposed to be rare. I have investigated the genetic mating system of the Black-headed Gull in which, as in seabirds and most other long-lived and socially monogamous birds, extra-pair paternity (EPP) is typically rather infrequent. The parentage of 79 chicks from 30 broods was determined based on the analysis of six microsatellite markers. In this study population, evidence of allelic inconsistencies between putative parents and chicks as a result of both EPP and intraspecific brood parasitism (ISBP) was found in 43% of nests . Extra-pair paternity was detected in 33% (10/30) of broods, and 20% (16/79) of all nestlings were sired by extra-pair males. In addition, 9% (7/79) of chicks out of five nests (17%) were not the offspring of either member of the pair, indicating ISBP. These findings reveal a moderate rate of ISBP and a high rate of EPP in the Black-headed Gull compared with other related species and shows that the Black-headed Gulls successfully participate in extra-pair copulations
Conspecific brood parasitism in birds occurs when a female inserts her egg into the clutch of her own species. If successful, i.e. the parasitic egg is accepted by the host, then the host female or pair rears the offspring of the parasite. In the present study, we studied natural conspecific brood parasitism in Black-headed Gulls (Larus ridibundus), and conducted series of the experiments with mimetic (conspecific) and non-mimetic (conspecific painted light blue) eggs to explore responses of the tested pairs towards these alien eggs. The natural parasitism rate was 10% and the probability of being parasitized significantly increased with nest density. Experimentally parasitized pairs rejected both types of experimental eggs at a similar rate: 14.3 % for mimetic and 25.5% for non-mimetic within 2 days. Non-mimetic eggs were more selectively rejected than mimetic eggs. The relationships between the probability of egg rejection (dependent variable) and predictor (independent) variables were examined by fitting generalized linear models. Contrast and intraclutch variation in ground color and spotting pattern and the volume of the egg had no significant effect on rejection behavior in either non-mimetic or mimetic eggs. However, nest density significantly positively affected rejection behavior of the Black-headed Gulls in both non-mimetic and mimetic treatments.
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