Contrasting brood sizes in Common and Arctic Terns: The roles of food provisioning rates and parental brooding

Arctic Terns (Sterna paradisaea) and Common Terns (S. hirundo) are similar in many aspects of their breeding ecology, but Common Terns generally lay t hree eggs per clutch whereas Arctic Terns lay two. In our study, Common Ter ns had a higher rate of food delivery and energy supply to the nest and hig her nest attendance, indicating that they made trips of shorter average dur ation. This suggests that the number of chicks raised by these two species was primarily limited by the rate at which parents could supply food. Howev er, estimated daily metabolizable energy intake of chicks was about 30% hig her in Common Terns than in Arctic Terns. Common Tern chicks apparently spe nt a higher proportion of daily energy intake on maintenance of body temper ature. It remains unknown whether this difference was because Common Tern p arents could not brood three chicks as effectively as Arctic Terns brooded two or because the energy requirements for heat production in the third-hat ched Common Tern chick were particularly high. If brooding did play a less important role in the energy budgets of Common Terns, the number of chicks that Arctic Terns could raise may have been limited not only by the rate at which parents could supply food to the nest but also by the requirements o f chicks for brooding. We suggest that more detailed studies on the role of brooding constraints in limiting brood size in these species are required to clarify this matter.