Proximate Costs of Competition for Nectar

Food competition among coexisting nectarivorous birds is conspicuousand often intense, affecting patterns of flower choice, dailybehavior budgets, and timing of successful reproduction. Exploitativecompetition involves loss of accumulated nectar to other individualsthat visited a flower first. Preliminary data support the useof Poisson models of the frequencies of point-source visitationand overlap for determining the probabilities of actual competitiveevents. Nectar losses from monitored flowers can be estimatedin terms of time intervals between visits weighted by flower-specificnectar production and bird-specific nectar removal capabilities.Foraging time budgets then provide a meaningful common denominatorfor assessing the impacts of competitive nectar losses, becausecompensatory increases in foraging time are required to maintaina balanced energy budget. Flexibility in foraging time budgetsmade possible by high efficiency foraging and predictably lowcompetitive losses may be an important determinant of reproductivetiming and success in nectar feeding birds. Aggressive displacement of competitors and territorial defenseof flowers are common forms of interference competition in nectar-feedingbirds. Aggression has definable caloric costs that ultimatelymust relate to caloric gains. Defense of flowers increases theaggressor's exclusive use of nectar, increases the predictabilityof a nectar supply, and increases the average amount of nectarobtained per flower. Simple cost-benefit models of territorialitydefine conditions when net benefits of territoriality are greaterthan those of alternatives.