Аннотация:Food limitation on reproduction during the nestling stage was experimentally tested in a Finnish population of Eurasian Kestrels (Falco tinnunculus). Nests were provided with supplemental food from hatching to fledging for three years (1992, 1993, and 1995), which differed in the natural abundance of voles, the Eurasian Kestrel's major prey. These supplemented nests were compared to nonsupplemented control nests using the following variables: (1) parental effort, (2) parental condition, and (3) fledgling number and quality. Female parents responded to food supplements by decreasing their hunting effort (estimated as the percentage of time spent flight-hunting) and prey delivery rate, whereas the hunting effort and/or prey delivery rate of males did not change. Females at supplemented nests were heavier than those at control nests, but the body mass of males was not affected by feeding. Supplementary food increased the number of fledglings per brood not only in years of low, but also in years of relatively high natural prey density. We conclude that even at high levels of natural food abundance, the reproductive output of kestrels is food-limited during the nestling period. In addition, there appear to be intersexual differences in the adjustment of parental effort. While females adjusted parental effort to male provisioning and offspring requirements, males did not change parental effort within a season, suggesting that male parental effort is fixed at a level where male survival is not jeopardized.
