Food supply has been acknowledged as one of eight major external factors regulating the sexual cycles of birds (Marshall 1961). Several hypotheses have been advanced to explain the role of food supply as an ultimate factor regulating breeding (Marshall 1951; Lack 1954, 1968; Wynne-Edwards 1962; and others). Another potential influence of food is its being a proximate stimulus to breeding. When certain foods become available they may act as a stimulus as the female requires them to meet her dietary needs during egg formation. Lack (1966a: 24) suggested that the average date of laying by the Great Tit (Parus major) probably resulted from a correlation between spring temperatures and the time of appearance of the insect foods the adult females need to form eggs. He also cited other passerines whose breeding was affected by food supply available to the female. In waterfowl it has been suggested that laying females require invertebrate foods (Moyle 1961, Leitch 1964). If true, this implies that the timing of laying is influenced by those environmental changes that affect food supply.

The Pintail (Anas acuta) lives primarily on plant foods during much of the year (Martin et al. 1951); thus a study of feeding ecology during the nesting season provided an opportunity to evaluate the significance of an invertebrate food source to females during the period of egg formation. Marshall (1951) stated that essentially vegetarian species appear compelled to switch at least partially to a heavier protein diet when feeding their developing young. Production of a clutch of eggs, like tissue growth in the young, requires a special dietary need that presumably should be reflected in the diet of vegetarian species during the period that eggs are being formed. This paper describes the diet of the female Pintail prior to, during, and after laying and discusses the impact of certain environmental and physiological changes on Pintail breeding.