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Floral nectar is offered by plants to animals as a reward for pollination. While nectar is typically a clear liquid containing sugar and trace amounts of amino acids, colored nectar has evolved in several plant families. Here we explore the functional significance of the phenolic compounds that impart a dark brown color to the nectar of the South African succulent shrub Aloe vryheidensis. Flowers of this aloe are visited for their nectar by a suite of short-billed birds that are occasional nectarivores, including bulbuls, white-eyes, rock thrushes, and chats. Dark-capped Bulbuls were more likely to probe model flowers containing dark nectar than those containing clear nectar, suggesting a potential signaling function for dark nectar. However, the main effect of the phenolics appears to be to repel "unwanted" nectarivores that find their bitter taste unpalatable. Nectar-feeding honey bees and sunbirds are morphologically mismatched for pollinating A. vryheidensis flowers and strongly reject its nectar. However, the frugivorous and insectivorous birds that effectively pollinate this aloe are seemingly unaffected by the nectar's bitter taste. Thus the dark phenolic component of the nectar appears to function as a floral filter by attracting some animals visually and deterring others by its taste.

Sunbirds (Nectarinia spp.) feeding at the East African mint Leonotis nepetifolia en- counter great variations in the nectar contents of flowers blooming in dense fields. The dispersion patterns of nectar are attributable to nectar removal from some flowers by the sunbirds themselves in earlier foraging and also to intrinsic floral variations. The problem facing the foraging sunbird is to increase its foraging efficiency (net energy gain per unit time) by avoiding recently visited, empty flowers and by visiting flowers with greater than average nectar volumes. Sunbirds patterned their foraging in 3 major ways. First, they used initial flowers probed on an inflorescence as an assay of what the rest of the flowers in that inflorescence contained and they rejected inflorescences with little nectar. Such rejection increased nectar intake per flower by as much as 15%. Second, territorial sunbirds preferentially fed at unvisited inflorescences, increasing nectar intake 25% relative to random foraging. This was accomplished at least in part by foraging at different heights on successive foraging bouts. Third, flight distances to the next flower changed in response to immediate reward levels in some species but not in others. In general, sunbirds feeding at Leonotis responded less to reward levels by differential turning and movement than some other organisms, possibly reflecting different prey distributions or boundary constraints on their foraging.

A series of 2- to 3-h observations of two breeding male Malachite Sunbirds (Nectarinia famosa) territorial at Aloe gramninicola flowers was made in the highlands of Kenya. Average time budgeting, and hence energy budgeting, varied little between males, but each male showed significant differences in time and energy budgeting with stage of the nesting cycle. Empirical regression equations were used to estimate the total energy intake from nectar and the average caloric intake per flower visited per observation period. Male 1 generally took in more energy from nectar than it spent during an observation period; Male 2 regularly took in somewhat less energy than it expended. Both males probably supplemented energy intake from nectar with energy from insects. Multiple regression analyses supported the idea that the percentage of time a male spent foraging was primarily a function of (1) the amount of energy obtained per flower on the average during the observation period, and (2) the amount of time and hence of energy that was spent in chasing other birds from the nectar sources being defended and from the vicinity of the female while she was nest-building. There was no relation between percent time foraging and (1) the ambient temperatures during the observation period, or (2) the amount of time spent in flight activities other than chases. It is suggested that these males made relatively short-term adjustments in their foraging time as a function of energy intake and to some extent expenditures. However, the relation of foraging time to chase time may reflect also the impact of intruders on the energy content of the territory of the male.