WHERE DOES A FRUIT-EATING BIRD DEPOSIT MISTLETOE SEEDS? SEED DEPOSITION PATTERNS AND AN EXPERIMENT

Abstract

The distribution of desert mistletoes (Phoradendron californicum) among mesquite (Prosopis velutina) hosts is significantly aggregated. We hypothesized that the aggregation of mistletoes among hosts is produced by the pattern of seed rain generated by seed dispersers. We tested whether frugivorous Phainopeplas (Phainopepla nitens) foraged preferentially, and hence deposited mistletoe seeds disproportionately, in already parasitized mesquites. We found that Phainopeplas favored parasitized tall trees as perching and feeding sites and deposited mistletoe seeds disproportionately on trees with these characteristics. To assess experimentally the effect of the presence of mistletoes on seed deposition, we removed mistletoes from host trees. Before mistletoe removal, seed deposition was equivalent and temporally correlated in pairs of control and removal trees. After removal, deposition was lower into removal trees than into unmanipulated trees. Although mistletoe removal resulted in lower seed deposition, it did not eliminate it. We inferred that seed deposition into parasitized hosts can result from deposition of seeds originating from within a host and from other infected hosts. We conclude that the response of seed-dispersing birds to mistletoes leads to disproportionate seed deposition into already parasitized trees and that the heterogeneous distribution of seeds among hosts created by birds contributes to the clumped distribution of mistletoes among hosts.