From the 1980s, Mediterranean shags Phalacrocorax
aristotelis desmarestii have become regular
summer visitors in the Gulf of Trieste (N-E Italy), as postbreeding
movements from Croatian breeding colonies. To
characterize such a recent habit and to explore diving
optimality models, we investigate foraging strategies and
diving patterns at diVerent depths, during breeding and postbreeding
seasons. Behavioural data were cross-checked
with the species’ diet. Shags foraged on and close to the sea
bed, with a prevalent anticipatory breathing strategy. In the
Gulf of Trieste, the shallow depths and low mobility of
prey allowed shags to use just the oxygen of the respiratory
tract, reducing the physiological stress for diving. In Croatia,
dive costs increased with depth and prey mobility,
resulting in a higher oxygen expenditure that involved also
respiratory stores. Such ecological and physiological
aspects characterize the Gulf of Trieste as an optimal area
for feeding and restoring from the costs of breeding season
incurred in Croatia and could be the basis of these
post-breeding movements.
