SENSORY CONTROL OF CLUTCH SIZE IN THE ZEBRA FINCH (TAENIOPYGIA-GUTTATA)

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.