Proximate Costs of Competition for Nectar

Abstract

Food competition among coexisting nectarivorous birds is conspicuous and often intense, affecting patterns of flower choice, daily behavior budgets, and timing of successful reproduction. Exploitative competition involves loss of accumulated nectar to other individuals that visited a flower first. Preliminary data support the use of Poisson models of the frequencies of point-source visitation and overlap for determining the probabilities of actual competitive events. Nectar losses from monitored flowers can be estimated in terms of time intervals between visits weighted by flower-specific nectar production and bird-specific nectar removal capabilities. Foraging time budgets then provide a meaningful common denominator for assessing the impacts of competitive nectar losses, because compensatory increases in foraging time are required to maintain a balanced energy budget. Flexibility in foraging time budgets made possible by high efficiency foraging and predictably low competitive losses may be an important determinant of reproductive timing and success in nectar feeding birds. Aggressive displacement of competitors and territorial defense of flowers are common forms of interference competition in nectar-feeding birds. Aggression has definable caloric costs that ultimately must relate to caloric gains. Defense of flowers increases the aggressor's exclusive use of nectar, increases the predictability of a nectar supply, and increases the average amount of nectar obtained per flower. Simple cost-benefit models of territoriality define conditions when net benefits of territoriality are greater than those of alternatives.