Pellets of Cape Cormorants as Indicators of Diet
Pellets of Cape Cormorants (Phalacrocorax capensis) are cast daily and can be used to determine prey species through identification of otoliths and other indigestible parts. Experiments in captivity indicate that some otoliths may be partially or entirely digested, making it impossible to estimate the original size or number of fish ingested per meal or per day. Birds of various species regurgitate casts or pellets of indigestible material (Knight 1964). Pellets of cormorants could be useful
... could be useful for studying diets of these birds because sampling is non-destructive and large samples can be obtained with little disturbance to the birds (Ainley et al. 198 1). Crystals of fish eyes taken from pellets might be counted to estimate daily food intake (Jordan 1959, Schlatter and Moreno 1976) and otoliths could be useful in identifying the species and sizes of fish consumed by the birds. However, certain aspects of pellet production require investigation before pellets can be used for either quantitative or qualitative studies. How frequently are pellets cast? Does a pellet (accurately) reflect a meal or a bird' s daily consumption of food? Do otoliths remain unchanged while residing in a bird' s stomach, so that otoliths can be used to estimate the sizes of the fish eaten? How long do otoliths persist in a bird' s stomach? We report here on studies designed to answer these questions, using information obtained from pellets produced by captive Cape Cormorants (Phalacrocorax capensis).