Sensory control of clutch size in the Common Swift Apus apus

Abstract

Clutch size varies among individuals in most bird species. A widespread assumption is that such variation results from variable timing in the disruption of ovarian follicular growth that brings, with a few days' lag, egg-laying to an end. Currently, there is empirical evidence that this is the case in Blue Tits Cyanistes caeruleus but not in Zebra Finches Taeniopygia guttata, in which the timing of follicular disruption has been shown to be invariant. Here, I investigate clutch size regulation of Common Swifts Apus apus. Using experimental egg removal, I show that, assuming the female gets enough egg contact, the determination of clutch size occurs at noon solar time on the day the first egg of a clutch is laid. In spite of individual variation in clutch size, there was no hint of any variability in the timing of clutch-size determination. In this species therefore, the timing of the signal disrupting follicular growth appears invariant. The physiological mechanism that controls clutch size is discussed, including the existence of an endogenous circadian clock and potential zeitgeber, the developmental range of clutch size for the species, the sensory nature of the input that triggers follicular disruption, and the stress of laying extra eggs.

Mechanisms of heterochronic change and stasis for clutch size in swifts (Apodiformes)

“…In birds, the physiological mechanism controlling clutch size can be decomposed into two parts: a specific signal, or induction; and the acquired ability to respond to this signal, or competence (Haywood, ; Haywood, ). In indeterminate laying species, induction originates from the environment (e.g.…”

“…eggs in the nest causing tactile stimulation of the brood patch); in determinate laying species, it originates from within the organism (i.e. gene expression presumably coupled to an endogenous clock) (Haywood, ; Haywood, ; Haywood, ). In both cases, the induction is transient and reaches, presumably via the hypothalamus and the pituitary gland, the ovary.…”

“…In contrast, competence takes time to develop. Its timing is tightly controlled by an endogenous circadian clock (Haywood, ). Yet, once acquired, it only lasts until a successful induction takes place (Haywood, ; Haywood, ).…”

“…Lack, ; Boyce & Perrins, ; Saether, ). They represent excellent bird families for this type of study because: (1) diversity of clutch size is extensive, ranging from a single egg to 5–6 eggs per clutch (see Table for a phylogenetic perspective); (2) some species, indifferent to the general increase in clutch size with latitude, provide remarkable cases of evolutionary stasis (see below); (3) all their constitutive species share the same exclusive reliance on flying arthropods for food (Lack, ; Chantler, ); and (4) in one species, the common swift Apus apus , the physiological mechanism controlling clutch size has been the subject of detailed investigation in relation notably to the concepts of induction, competence, and determination (Haywood, ). In an attempt to test the model, I will examine two predictions in light of both observational and experimental data available in swifts.…”

Solar noon and tactile cues synergistically regulate clutch size: a new approach to investigations of avian life‐history theory

“…Variation in clutch size is assumed to depend on the variable timing of ovarian follicular growth, which subsequently produces variation in the hormonal feedback that permits successive ovulations to occur (Haywood 2013). When hormonal surges occur outside the permissive zone, ovulation is prohibited (Fraps 1955, Johnson & van Tienhoven 1980, Etches et al 1984, Underwood et al 1997.…”