Sensory Control of Clutch Size in the Zebra Finch (Taeniopygia guttata)

Abstract

A!tSTRACT.-An analysis of the mechanism responsible for controlling clutch size was carried out in the Zebra Finch (Taeniopygia guttata) by manipulating the number of eggs present in the nest during laying. Sensory cues involved were also investigated. Stimulation from the eggs present in the nest during the second and third day of the egg-laying period determined how females cease to lay in this species. Disruption of ovarian follicular growth was triggered on the third day of laying, 6 _+ 1 h after the onset of light (14L:10D), by tactile stimulation. Contact with eggs also was required to induce the onset of sensitivity to the stimulus responsible for ending laying. The hypothesis that individual variations in clutch size (four to six eggs) are caused by early disruption of follicular growth for smaller clutches versus late disruption for larger clutches was tested. The timing of follicular disruption showed no significant variation between females (n = 12) in relation to clutch size. There is good evidence that this result, established in domesticated Zebra Finches, holds true for wild Zebra Finches as well. Individual variation in clutch size resulted from the variable number of growing follicles that, once follicular disruption has been triggered, were able to complete the rapid-growth phase and ovulate. CONSIDERABLE PROGRESS has been made in our understanding of the evolution of clutch size since Lack (1947) first attempted to explain why birds lay the number of eggs they lay. Although it has been known for quite some time that clutch-size control mechanisms may differ between species (Cole 1917), how birds control the size of a clutch is still a very little known aspect of their breeding biology. Some species, classified as tactile indeterminate or semideter-minate layers, have been shown to rely on eggs present in the nest during the laying period to cease laying whereas others, classified as deter-minate layers, do not need such stimulus (Hay-wood 1993a). The role of a contact between the female and her eggs in stopping laying has been substantiated, through egg-removal experiments , in a number of species, mostly nonpas-serines (for a review, see Haywood 1993a). Studies carried out in gulls have demonstrated that the contact between the female and the first egg of a clutch results in disruption of ovarian fol-licular growth (growing follicles becoming atretic instead of ovulating) and then, after a few days, cessation of egg laying (Paludan 1951, Weidmann 1956). The aim of my study was to analyze in the Zebra Finch (Taeniopygia guttata) how females control their clutch size by manipulating the number of eggs present in the nest during laying. Experiments were carried out to determine: (1) whether the Zebra Finch is, like the few passerine species studied so far (Haywood 1993a), an indeterminate layer; (2) if yes, the number of eggs and the duration of contact required to induce the female to cease laying; (3) the type of stimulus perceived by the female that might be visual, tactile, olfactory, or auditory ; and (4) the timing of the stimulus responsible for the ending of laying. Given that clutch size varies in this species, usually from four to six eggs, an attempt has also been made to uncover the mechanism responsible for individual variation in clutch size. If a contact between the female and her eggs is involved, it may be hypothesized that in females laying smaller clutches laying might be inhibited sooner by the eggs already present in the nest than in females laying larger clutches. This hypothesis was tested by determining for each of a dozen females the characteristic, average size of several successive clutches and the precise timing of disruption of ovarian follicular growth. METHODS Zebra Finches bred in captivity for many generations were kept in indoor cages (35 x 40 x 45 cm). All pairs were maintained at about 20ø(/and under a photoperiod of 14L:10D; with light on at 0800. Pairs were provided with fresh water, seeds, egg food, grit, and cuttlebone ad libitum. In addition, salad leaves for 778