Numerous studies of predatory birds worlwide report dietary proportions based on analyses of large numbers of pellers or prey remains. Such analyses are often severely biased, hence strictly unquantifiable, because some prey remain are more conspicuous or persitent than others We investigated this bias for the bird-and micromammal-eating African Marsh Harrier (Circus ranivorus), using an essentially independent measure of diet, observed prey deliveries to the nest.