ARE BIRD-CONSUMED FRUITS COMPLEMENTARY RESOURCES

We conducted field experiments in 1994 and 1995 to determine whether f leshy fruits consumed by North American, migrant passerines are antago nistic, complementary, or perfectly substitutable resources. Joint con sumption of resources that are antagonistic results in reduced fitness , whereas joint consumption of those that are complementary resources results in increased fitness, than would be predicted from consumption of a linearly weighted sum of the two (or more) resources. Joint cons umption of perfectly substitutable resources results in fitness equal to that predicted by consumption of the linearly weighted sum of the r esources. Of ten pairs of fruit species tested, eight exhibited resour ce complementarity (sign test, P < 0.05); however, the strength of com plementarity varied among the species pairs. In the two comparisons in which evidence for complementarity was not found, most fruit consumpt ion appeared to have been accomplished by seed predators rather than t rue frugivores (house finches, Carpodacus mexicanus, in one case, and eastern chipmunk, Tamias striatus, in a second). In a replication of t he second comparison, most consumption of one fruit species was due to a flock of starlings (Sturnus vulgaris), a species lacking sucrase ac tivity (which may help explain the result). We suggest that resource c omplementarily is generally common among fruits, which may help explai n why so few frugivores are found to specialize on only a single or ev en a small number of fruit species. Resource complementarity has impor tant implications for the ecological and evolutionary interactions of avian frugivores and fruit-producing plants. The biochemical and physi ological bases of complementarity are unknown, but could involve nutri ents, secondary metabolites/toxins, or both.