Do the contents of Barn Owl pellets accurately represent the proportion of prey species in the field?

Prey composition of Barn Owl (Tyto alba) pellets from northwestern Negev, Israel, was examined. The 414 individual specimens of mammals represented by 256 pellets comprised: 9.2% Meriones sacramenti, 41.1% M. tristrami, 8.2% Gerbillus andersoni, 40.1% Mus musculus, and 1.4% Crocidura suaveolens. The pellets also contained some remains of insects, small specimens of the snake Eryx jaculus, and two passerine birds. We tested whether the observed distribution of prey species in the 256 pellets could be obtained if owls hunted at random. Direct calculation and simulations indicate that more single-species pellets contained large mammals than would be expected from random sampling. In simulation of owls sampling at random from the database, the distribution of 'pellets' containing 1, 2, 3, or more prey items was similar to the observed distribution only when a cumulative weight limit for pellet ejection was set at 80-100 g. Even when Barn Owls do not hunt some species preferentially, the contents of the pellets may be biased towards larger prey. This result should be taken into consideration when accumulated pellets are used in ecological and paleontological studies to approximate the distribution of mammal prey in real communities, present or past.