Effect of Supplemental Food on the Breeding of Blue and Great Tits in Mediterranean Habitats

To study the effect of food availability on the onset of reproduction, clutch-size and breeding success in the Blue Parus caeruleus and Great Tit P. major, we experimentally supplied food on parts of a holm oak study site in Mediterranean France during two consecutive years and compared the birds' responses with the natural patterns in holm and downy oak habitats. In the Blue Tit, the mean stage of nest construction was significantly more advanced in the experimental than in the control area in one year; the mean laying date was significantly earlier in the experimental area in both years (difference 6 d in 1986, and 5 d in 1987). Results for the Great Tit showed the same but non-significant trend. For both species, no significant differences in clutch-size were found between experimental and control birds. Supplemental feeding did not increase the Blue Tit's breeding success which was even significantly lower in the experimental than in the control area in 1987. In that year, the experiments apparently advanced laying to the extent that nestlings were present prior to the food peak. We conclude that food availability does not limit laying but rather acts as a stimulus triggering it. However, our results do not exclude that feeding conditions in early spring may also influence the onset of laying by enhancing the physiological condition of the pre-laying female.