Reflections of a specialist: patterns in food provisioning and foraging conditions in Sandwich Terns Sterna sandvicensis

Highly specialised piscivorous seabirds, like Sandwich Terns Sterna sandvicensis, have limited capacity to switch to alternative prey species when the availability of a particular prey species is low. Therefore, variations in the diet of such species are likely to reflect fluctuations in food availability. We studied food provisioning of Sandwich Tern chicks on Griend in 1992-98. About 75% of the fish brought to the colony were eaten by the chicks. The rest was lost, mainly to robbing Black-headed Gulls Larus ridibundus. Parents met the increasing energy demands of the growing chicks by adjusting prey size, rather than increasing the rate of prey transport to the chicks. Distinct patterns in food transport rate, diet composition and prey size were associated with weather conditions and diurnal or tidal rhythms. Food transport to the colony was severely curtailed by strong winds, but was also relatively low at low wind speeds. Wind speed also had a large impact on prey size and diet composition, with a decreasing proportion of Clupeidae brought to the chicks as foraging conditions became worse. Distinct diurnal rhythms in food transport coincided with diel vertical migration patterns in Clupeidae and Ammodytidae. Clupeidae were mostly brought to the colony early in the morning and late in the evening, while the transport of Ammodytidae was highest around noon. Tidal patterns in food delivery rate were probably related to tide-specific foraging areas used by the terns. A fish monitoring programme showed considerable variation in food abundance within the foraging area of the terns. Especially Clupeidae had a patchy distribution and most clupeids were caught in the coastal areas around Vlieland. In accordance to the pattern found in the colony, Clupeidae caught in 1996 and 1997 towards fledging of the chicks and just after hatching of the chicks in 1998 were relatively small.