Comentaris sobre els desplaçaments erràtics de la població catalana de gavià argentat "(Larus argentatus michahellis)"

Abstract

The erratical displacements of the Catalan population of the Herring gull (Larus argentatus michahellis) are analysed and discussed, using country observations and 44 recoveries of the 3,104 pulls ringed in the Medes Islands (Girona) and the Ebre delta (Tarragona) colonies. The seven colonies that constitute this population, which is of about 8. 140 couples, are described. It is exposed that a sizable number of the juveniles (22 % according to our recoveries) travels to the French Atlantic Ocean, between the Spanish Basque Country and the Loire delta, from July to November. The farthest recovery occurs 750 km NW from the birth locality. The rest of the juveniles remains near the breeding colonies, entering in the continent and following rivers over a maximum of 160 km from the coast. These movements show a clear parallelism, in space and time, with those of the French Mediterranean population, which make us consider the Franco-catalan mediterranean population, as an homogeneous dispersions nucleus. Although ringing shows the existence of a native population crearly sedentary, on the other side it hasn't been proved that the juveniles gone to the French Atlantic return to the birth areas. The possible penetration routes to the Atlantic of the Franco-catalan population, Ere discussed, the most apropiated, seems the course of the Aude-Canal du Midi-Garonne, probably also used by other sea species (Gavia sp.). Important post-nuptial concentrations are seen out of the Medes Islands breeding area, which seems to indicate that the Franco-catalan population performs moulting migrations. The invernal bird counts of the French and Catalan-levantine Mediterranean Sea are compared, observing that although both breeding populations are quite similar, the invernal populations are very different. The typology and motivation of these movements are discussed, indicating that they don't always have to be considered as erratic