The Effect of Extra Food on Fitness in Breeding Carrion Crows

Food limitation during the nestling stage was experimentally investigated in an urban population of Carrion Crows. Parents were offered supplemental food during the nestling period and the following variables compared with those from non food-supple- mented pairs: (1) nesting success, (2) fledgling number, (3) fledgling mass and linear body size, (4) proportion of fledglings resighted (a) after transition to independence from the parents, and (b) after the first winter postfledging, and (5) the probability that a fledgling became a breeder. Food-supplemented pairs had a higher nesting success and produced more fledglings. The fledglings of food-supplemented pairs were heavier and bigger in tarsus length than controls, but experimental and control fledglings were seen in equal proportions both after transition to independence and after the first winter. The production of potential breeders was increased ninefold in food-supplemented pairs. The study demonstrates that in this urban habitat food limitation causes low fitness not through lowered survival, but through the production of a low number of potential breeders.