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 timing for the onset of incubation. We measured fecal estradiol-17beta (E) and progesterone (P) in laying canaries to better understand how onset of incubation might regulate clutch size. Both E and P rose sharply to maxima 1 day before the first egg was laid. Thereafter, E steadily declined, but P remained high through 2 days after the first egg was laid, after which both hormones had returned to low levels. Clutch size did not explain variation in E or P output during the laying cycle. When analyzed with respect to onset of incubation, E and, to a lesser extent, P dropped significantly on the day incubation began, irrespective of whether or not females had finished ovulating. We suggest that factors initiating incubation also cause the decline in E production by small follicles, which in turn may inhibit yolk sequestration in large follicles. Further experiments in which onset of incubation is manipulated may reveal the mechanisms by which this behavior regulates clutch size and reproductive output.