FOOD LIMITATION ON BROOD SIZE - EXPERIMENTAL-EVIDENCE IN THE EURASIANKESTREL

Food limitation on reproduction during the nestling stage was experime ntally tested in a Finnish population of Eurasian Kestrels (Falco tinn unculus). 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 usin g the following variables: (1) parental effort, (2) parental condition , and (3) fledgling number and quality. Female parents responded to fo od supplements by decreasing their hunting effort (estimated as the pe rcentage 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 nest s, but the body mass of males was not affected by feeding. Supplementa ry 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 repr oductive output of kestrels is food-limited during the nestling period . In addition, there appear to be intersexual differences in the adjus tment of parental effort. While females adjusted parental effort to ma le provisioning and offspring requirements, males did not change paren tal effort within a season, suggesting that male parental effort is fi xed at a level where male survival is not jeopardized.