Prey Selection during Courtship Feeding in the Common Tern

Abstract

Courtship feeding was originally believed to be important only in the establishment and maintenance of pair bonds between birds (Lack 1940, Hinde 1964). There is now no doubt that in some species it must also be important in the nutrition of the female, particularly during the period of egg formation (Royama 1966, Brown 1967, Krebs 1970). Direct evidence of this has recently been provided for the Common Tern Sterna hirundo, (Nisbet 1973). Nisbet showed that the total fresh weight of the clutch and the fresh weight of the third egg in particular were correlated with the amount of food given to the female. Fledging success from third eggs was positively related to the weights of those eggs. The amount of food brought to the female during courtship feeding was therefore an important factor influencing breeding success. Common Terns usually only carry one prey item at a time (Taylor 1975) so that each item fed to the female on the nest territory requires a separate journey to and from the feeding ground. This obviously must involve the male in a large additional expenditure of time and energy which he may not always be able to meet. Any factors that help to minimize this load and hence ensure maximum food supply to the female would be strongly selected for. This paper gives a comparison of the prey items eaten by male Common Terns and those carried to the female and shows one way in which the male is able to minimize his time and energy expenditure. Methods