Daily estradiol and progesterone levels relative to laying and onset of incubation in canaries

Female birds may optimize reproduction by modifying clutch size and the tim ing for the onset of incubation. We measured fecal estradiol-17 beta (E) an d progesterone (P) in laying canaries to better understand how onset of inc ubation might regulate clutch size. Both E and P rose sharply to maxima 1 d ay before the first egg was laid. Thereafter, E steadily declined, but P re mained high through 2 days after the first egg was laid, after which both h ormones had returned to low levels. Clutch size did not explain variation i n E or P output during the laying cycle. When analyzed with respect to onse t of incubation, E and, to a lesser extent, P dropped significantly on the day incubation began, irrespective of whether or not females had finished o vulating. We suggest that factors initiating incubation also cause the decl ine in E production by small follicles, which in turn may inhibit yolk sequ estration in large follicles. Further experiments in which onset of incubat ion is manipulated may reveal the mechanisms by which this behavior regulat es clutch size and reproductive output. (C) 1999 Academic Press.