Onset of Incubation in Yellow Warblers: A Test of the Hormonal Hypothesis

Abstract

Mead and Morton (1985) suggested that asynchronous hatching for birds in which only the female incubates is caused by a hormonal surge associated with the ovulation of the last ovum. This hormonal surge is believed to inhibit further ovulation and simulta-neously stimulate females to initiate full incubation. Thus, the advantages, if any, accrued from asynchronous hatching are merely epiphenomena. The hormonal hypothesis predicts that regardless of clutch size, full incubation is initiated with the laying of the penultimate egg. We also predict from the hypothesis that incubation attentiveness during egg laying will be similar in females tending either four- or five-egg clutches, especially on the day the penultimate and last eggs are laid, hatch spreads will be similar in four- and five-egg clutches, and the addition of eggs during egg laying will not induce increased incubation attentiveness. In a three-year study (1988-1990) on Yellow Warblers (Dendroica petechia) nesting at Delta Marsh, Manitoba, we found that incubation attentiveness during egg laying was similar between females that produced four- and five-egg clutches, but full incubation was not initiated prior to the last egg being laid. Despite apparently similar incubation attentiveness during egg laying, hatch spreads differed significantly between four- and five-egg clutches. Incubation attentiveness was increased experimentally by adding eggs during early egg laying. We suggest that initiation of full incubation is not rigidly controlled by hormonal changes associated with the laying of the penultimate egg. Therefore, asynchronous hatching of the penultimate and, especially, the last egg does not require full incubation.