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.