La Mouette mélanocéphale Larus melanocephalus comme aide à la Mouette rieuse L. ridibundus pendant sa reproduction

In both 1995 and 1996 an unpaired male Mediterranean Gull was observed helping a pair of Black-headed Gulls during breeding, near Antwerpen (northern Belgium). Two different birds were involved as one was in adult plumage, the other second summer. In 1995, the helper assisted greatly; in nest construction, breeding, care and feeding of the two chicks, one of which survived at least to the fledging stage. In 1996 the co-operative trio broke up during nest-construction and breeding stage. The ealy break-up in 1996 was probably caused by the the arrival of three pairs of Mediterranean Gulls near the trio's nest. In 1995 the trio were surrounded only by Black-headed Gull nests (FIG. 1.). Due to the larger stature and especially the stronger territoral defence of the Mediterranean Gull, as compared to the Black-headed Gull, the former largely dominates on the breeding grounds. It was difficult to know whether the Mediterranean Gull was paired with a Black-headed Gull or only held a helping function, until they were seen as a trio. All partners of the trio were only presentt at the site together for 5.3% of observation time (TAB. I.). Proof of there being a trio took three hours of observation in 1995 and eight in 1996. Until now, interspecific helping behaviour hasn't been described for the Mediterranean Gull. Our observations may raise doubts as to whether true mixed Larus melanocephalus/L. ridibundus pairs exist. Within the Laridae, the Mediterranean Gull shows some unique courtship behaviour, the Rocking Long Call Sequence being the commonest. The complexity of this behaviour would suggest that hybridization between the two species would be very unlikely.