Arctic Terns (Sterna paradisaea) and Common Terns (S. hirundo) are similar in many aspects of their breeding ecology, but Common Terns generally lay three eggs per clutch whereas Arctic Terns lay two. In our study, Common Terns had a higher rate of food delivery and energy supply to the nest and higher nest attendance, indicating that they made trips of shorter average duration. This suggests that the number of chicks raised by these two species was primarily limited by the rate at which parents could supply food. However, estimated daily metabolizable energy intake of chicks was about 30% higher in Common Terns than in Arctic Terns. Common Tern chicks apparently spent a higher proportion of daily energy intake on maintenance of body temperature. It remains unknown whether this difference was because Common Tern parents could not brood three chicks as effectively as Arctic Terns brooded two or because the energy requirements for heat production in the third-hatched 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 of 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.
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Vol. 103 • No. 1