Role of extrinsic factors in the control of clutch‐size in the Blue Tit Parus caeruleus

Extrinsic factors that play a role in the control of clutch‐size in Blue Tits Parus caeruleus were investigated. The disruption of ovarian follicular growth, and hence the cessation of egg‐laying, is caused by a stimulus generated by eggs present in the nest. The timing of this disruption varies among females during the laying period and may happen as late as after the laying of the eighth egg of the clutch; the larger the clutch, the later the disruption of follicular growth. The timing of follicular disruption is also related to ambient temperature because the proportion of female Blue Tits that cease to lay increases when higher temperatures occur toward the end of laying. It is suggested that the use of temperature to control clutch‐size may be adaptive because the food peak on which parents rely to feed their chicks is likewise affected by temperature.