Although interspecific competition is of major importance in ecological and evolutionary theory1, there has been no critical study of whether indirect estimates of niche overlap accurately reflect the effects of competition on fitness as indicated by reproductive output. I report here that competition from jackdaws, Corvus monedula, significantly reduced reproductive output of magpies, Pica pica, due to starvation of nestlings and a high rate of nest predation. The competition coefficient as estimated from these direct effects (α = 0.43) was only slightly smaller than predicted by niche overlap values based on morphological similarity, habitat distribution and feeding technique (α = 0.58), or food composition (α = 0.54).