Clutch Size Determination in the Marsh Tit (Parus Palustris)

Abstract

By providing supplemental food to Marsh Tits (Parus palustris) from the day the first egg was laid, effects on clutch size could be directly attributed to food without the confounding effects on laying date. Females provisioned with additional food responded by laying, on average, on egg more than control females. Differences in territory quality did not influence this result because, on territories occupied in both years, one more egg was laid in the year of provisioning. Although the rate and amount of extra food provided were equal during the two experimental years, the resulting clutch sizes of provisioned females differed between years by more than one egg in absolute terms. Further, the mean clutch size of provisioned females declined seasonally in parallel to that of unprovisioned females. I conclude that clutch size in Marsh Tits is determined by a tactical decision based on declining values of offspring with the progress of the season. However, within the limits sets by this tactical decision, the availability of food during egg—laying also plays a role.