Foraging Strategies and Energetics of East African Sunbirds at Mistletoe Flowers

Abstract

Sunbirds (Nectariniidae) feeding on nectar of the mistletoe, Phragmanthera dshallensis, in central Kenya encounter two types of flowers. Closed flowers, which have not been visited previously and must be opened by the feeding bird, contain more nectar and require more feeding time than open flowers. Open flowers result from the visitation and explosive opening of closed flowers. Closed flowers are rarer overall than open flowers, although occasionally a local area may have a majority of closed flowers. The different-sized sunbird species tend to take different proportions of open and closed flowers; generally there is a positive relation between bird size and percentage of closed flowers taken. We examined benefits and costs for each bird species feeding in local patches of mistletoe. The benefits were the same for each species. The time costs varied inversely with bird size. These differences were reduced by the lower energetic costs of the smaller species. For all species the caloric benefit to cost ratio (= foraging efficiency) increased with increasing proportions of closed flowers visited. However the foraging efficiency difference between 0% and 100% closed flowers was greatest for kilimensis, indicating that kilimensis gained more by preferentially moving to patches with high proportions of closed flowers. The data on percentage closed flowers actually taken indicate that those species which should derive greater energy benefits by taking higher proportions of closed flowers tend to do so. Complete specialization on closed flowers, however, apparently is prevented by increasing search costs within and between patches. The sunbird foraging behavior is consistent with predictions based on maximizing foraging efficiencies as a function of relative energy value of the two flower types.