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Sensory and Hormonal Control of Clutch Size in Birds

The control of clutch size is examined is several taxa of birds and the concept that species can be classified as determinate or indeterminate layers is reeveluated. Determinate layers are defined as species in which extrinsic factors perveiced by the female, such as eggs present in the nest, are not involved in determining either the number of large yolky follicles produced by the ovary or the number of follicles ovulated (e.g., albatrosses, auks and pigeons). In some species, the number of large yolky follicles produced is unrelated to extrinsic factors, but contact between the female and her eggs usually reduces the number of follicles ovulated and hence the clutch size. These species are classified as semideterminate layers (penguins). Among indeterminate layers, both the number of large yolky follicles and the number of follicles ovulated depend upon extrinsic factors. Three categories of indeterminate layers are recognized, depending upon the nature of the stimulus that stops egg laying. The first is tactile contact between the female's brood patch and the eggs; it is the most common. This category may be subdivided according to the number of eggs required to stimulate the brood patch. In some species egg removal leaving no egg in the nest is required for the female to lay extra eggs (contact with a Single egg sufficient: type S, Woodpeckers, plovers); other may lay extra eggs even if an egg is left in the nest (contact with Multiple eggs required: type M, kestrels, hawks, grebes). The second category of stimuli causing laying to cease, observed in parasitic birds, is probably the lack of suitable host nests. These visual cues trigger either reabsorption of all large yolky follicles (Discontinuous production of large yolky follicles: type D, parasitic cuckoos) or simply reabsortion of one or two large yolky follicles (Continuous production of large yolky follicles: type C, parasitic cowbirds). The third category, used by megapodes, could be thermal information derived from their nest mounds. Each category can be discriminated by observations on egg laying and appropriate egg removal experiments. Anatomical data from laying females are also a valuable source of information. The endocrine mechanism of clutch-size determination is still poorly understood; it has been linked to the onset of incubation behavior and in particular to a rise in prolactin levels. I argue that the rise in prolactin levels associated with incubation behavior might play a role in the determination of clutch size in tactile indeterminate layers, but not in determinate layers. Inhibition of gonadotrophin secretion constitutes another hypothesis that might be relevant whatever the mode of clutch-size control. Finally, a new hypothesis is proposed to account for the evolution of the control of clutch size in birds that relates determinate and indeterminate laying patterns to food availability at the time of egg formation, and variability of the best time for raising young.