Fruit-eating in birds and its nutritional consequences

Fruits comprise a large portion of the diet in many bird species. Several species switch diet composition seasonally, from a diet consisting almost entirely of invertebrates to one that comprises even exclusively fruits, and vice versa. The adaptive significance of these dietary shifts has rarely been investigated. The present article reviews some recent data revealing much evidence that seasonal frugivory is highly adaptive, and that fruits may be an adequate food to satisfy the birds' daily nutritional demands. There is further evidence that fruits may provide particular nutritional qualities to meet particular demands. Secondary plant metabolites considered to act as feeding deterrents seem to be less detrimental to avian frugivores than observed in many herbivores. The birds appear to be able to counteract the detrimental effects of secondary compounds taken with their fruit diets. In addition, there is evidence that secondary plant compounds may even stimulate food intake and metabolism in avian frugivores, although the mechanisms of counteradaptation need to be clarified.