Migration patterns of the Mediterranean Shag Phalacrocorax aristotelis desmarestii (Aves: Pelecaniformes) within the northern Adriatic Sea

摘要

Movements of Mediterranean Shags (Phalacrocorax aristotelis desmarestii, Payraudeau 1826) between Croatian breeding colonies and non-breeding areas such as the Gulf of Trieste increased consistently from the 1980s to become a migratory movement at the present time. In order to characterise the patterns of this migration and the behaviour of first-year Shags at their first migration, we analysed the recoveries of 812 birds that were colour-ringed at the most important Croatian breeding colonies in the northern Adriatic. Within a period of seven years, 568 ring-readings of 234 individual Shags were processed. Most sightings came from the Gulf of Trieste (43.0%) and the newly-discovered post-breeding area in Venice Lagoon (38.9%). Shags from the most distant colonies exploited mainly the Gulf of Trieste. The 48.3% of Shags sighted several times have been recorded in the same post-breeding area during subsequent years. This percentage increased to 93.4%, if we considered just the birds observed during successive years. Variations in the timing of migration within the post-breeding areas occurred and sustained the importance of the Slovenian coast during the return to breeding colonies. The ratio between first-year Shags and experienced Shags (immature and adults) was higher in Venice Lagoon and in general increased with the novelty of the site. A relevant portion of the Croatian breeding population moves to the northern Adriatic Sea after breeding. All the studied colonies concurred with this migration. We highlight a high fidelity to post-breeding sites, which reflects the quite sedentary nature of the species, and the role of first-year Shags in the discovery of new sites. We suggest that this migration is probably the later stage in a graded response to deterioration of the Shags' feeding grounds within the breeding area.