Fine-Scale Variation in Optimal Clutch Size in a Blue Tit Population

AbstractClutch size variation may be adaptive in bird populations if parents lay a clutch size that maximizes offspring recruitment. Brood size manipulation experiments allow for testing the optimization of clutch size by studying the effect on nestling body condition and fitness-related physiological traits. If the clutch size is optimized, parents rearing an enlarged brood size are expected to produce low-quality nestlings. Here, we tested this prediction in a population of blue tits (<i>Cyanistes caeruleus</i>), inhabiting two nearby oak forests (differing mainly in humidity) in which blue tits differ in clutch size. We manipulated the brood size (producing reduced, control, and enlarged broods) within each habitat and evaluated the immune system, oxidative status, telomere dynamics, and morphological parameters of the largest and smallest nestlings within broods. We also recorded parental feeding effort and nestling diet composition. Clutch size was higher in the more humid forest, and parents from the two habitats successfully raised the extra nestlings. However, the nestling body mass diminished in enlarged broods compared to control and reduced broods, while the structural size, telomere dynamics, immune system, and oxidative status did not vary with the manipulation treatments. In each forest, parents did not increase their feeding effort in enlarged broods, despite initial between-forest differences in clutch size. Our findings suggest that although blue tits were able to raise extra offspring, clutch size is optimized at the fine scale. Hence, blue tits seem to lay, in every habitat, the clutch size that guarantees the maximal production of high-quality fledglings.