The effectiveness of endozoochory in three avian seed predators

Abstract

The role of granivorous birds as agents of seed dispersal has been little explored and is poorly understood. We assessed the ability of three species of birds from a Central European agricultural landscape to disperse seeds of dry-fruited plants. We hypothesised that Grey Partridge Perdix perdix is a better seed disperser than either of two species of buntings—Yellowhammer Emberiza citrinella and Reed Bunting Emberiza schoeniclus—in terms of the number of intact seeds recovered from their droppings. Partridge droppings contained the highest number of intact seeds. Surprisingly, however, the number of intact seeds per 1 g of droppings was the highest in Reed Bunting, smaller in Grey Partridge and the smallest in Yellowhammer. Our findings suggest that the passage of intact seeds of dry-fruited plants through the digestive tract of seed-eating birds is most likely an effect of limited digestion, resulting from the intake of a large volume of seeds, a small part of which remains undigested. This effect could be magnified by the inclusion in the diet of some items of different digestibility (invertebrates or leaves). We suggest that non-standard dispersal of seeds with no adaptations to endozoochory by birds is a far more frequent and as yet under-appreciated phenomenon, which has potential ecological implications for the colonisation of new habitats/islands by plants. The ultimate elucidation of this process is extremely difficult and would require large sets of faeces to be examined.