Intercolony and seasonal differences in the breeding diet of European shags on the Galician coast (NW Spain)

The seasonal and spatial variations in the diet of the European shag Phalac rocorax aristotelis were studied during the breeding period, based on the a nalysis of 202 adults' pellets (February to June, 1995) and 30 regurgitatio ns of chicks (April, May and June, 1995 and 1996) taken from 2 colonies loc ated in close proximity to one another (<15 km) on the coast of Galicia (NW Spain; Islands of Cies and Ons). The diet of birds from the Cies Islands c onsisted mainly of sandeels (family Ammodytidae), which make up over 70% of the prey during all months. These fishes went practically unreported in a number of previous studies of fish communities in the region, probably due to the low catchability of trawl nets and the possibility that the habitats of sandeels where shags forage, shallow (<10 to 15 m) sandy bottoms, were not sampled adequately. Seasonal changes were found on the Island of Ons, w here in winter (February and March) the diet was based on gobids (family Go biidae) and sand smelts Atherina presbyter, while in spring the dominant pr ey were the Ammodytidae (in May and June they made up over 86%). The consum ption of sandeels established an increased similarity in diet between the c olonies throughout the breeding cycle, and they were the only prey whose ab undance was negatively correlated with the diversity of each pellet. This s tudy highlights the high plasticity in prey and feeding habitats (both pela gic and benthic with different types of substrates) used by the European sh ag. It should be noted, however, that the main prey during chick rearing ar e sandeels. Differences were found in the mean sizes of the different prey consumed, but the modal size for all of them throughout the season was arou nd 9 to 11 cm in total length. The range of sizes available in the environm ent is greater than the range chosen by the European shag, which suggests t hat this bird selects a narrow range of prey sizes.