This research will test the general hypothesis that a specialist frugivore (Phainopepla nitens) provides superior seed dispersal for desert mistletoe (Phoradendron californicum) than does a group of generalist foragers. The project will examine components of seed dispersal that can be expected to vary with the quality, measured in terms of costs incurred by the plant, of the dispersal agent. Such components of dispersal include proportion of seeds handled, survival of handled seeds, and survival of dispersed seeds. Complexities of the dispersal system, including impact of host tree species, seed clumping, and time of year on germination and seedling establishment will be examined experimentally. Because each of the components can be measured in terms of seeds wasted for each established seedling, researchers will quantitatively compare the cost of dispersal provided by a specialist with that provided by a group of generalists. This study will indicate how a dispersal mutualism actually influences plant reproduction. While seed dispersal by either specialists or generalists is technically mutualism, the study will distinguish strong from weak interactions between the birds and plants. This will be accomplished by examining the combined effects of fruit defense, seed dispersal, and seedling establishment for mistletoe reproduction.
