Living with gulls: The consequences for sandwich terns of breeding in association with black-headed gulls

We studied the feeding ecology of Sandwich Terns (Sterna sandvicensis) in the presence of kleptoparasitising Black-headed Gulls (Larus ridibundus) on the Isle of Griend, The Netherlands, between 1992 and 1998. About 30% of all of the food parents transported to the colony were lost, mainly through intervention of Black-headed Gulls. The gulls mainly took the larger fish, but showed no preference for the energetically more profitable herring. Apparently the gulls selected for prey length or visibility of the prey, rather than for energy content of the fish. The proportion of food robbed by the gulls increased with the age of the tern chicks and paralleled the increase in prey length Sandwich Tern parents brought to the colony. Kleptoparasitism showed a clear pattern with time of the day, tide and wind speed. During the first few hours of the day, almost no kleptoparasitism occurred, while robbery was high around 09.00 and at dusk. This bimodal pattern in kleptoparasitism might be related to the foraging activity of the gulls. A tidal effect on food loss was less pronounced, although kleptoparasitism was significantly higher during high tide, when foraging activity of gulls for other foods was low and the number of potential pirates in the tern colony was high. Wind strength had significant negative effects on the amount of food transported to the colony, while kleptoparasitism increased. Therefore, wind speed severely affected energy intake of the tern chicks and had strong negative effects on chick growth. During the first two weeks post-hatching kleptoparasitism was relatively low and had only small effects on chick growth, even under unfavourable weather conditions. From then on, however, the negative effects of kleptoparasitism on growth became considerable. Especially with strong winds, chick growth was severely affected. Sandwich Terns show several behavioral strategies in order to reduce the rate of food loss by the gulls and to minimise the effects on chick growth and survival.