In this study, Walt Koenig and I show that avian hoarders - birds that store their seeds for the winter - play an important role in mediating competition among oak species. California scrub-jays (Aphelocoma californica), high-quality seed dispersers of acorns, show up in large numbers on valley oaks (Quercus lobata) when no other acorns are available in the landscape. For the oaks, this means that a majority of their acorns is at least given a chance because jays cache their acorns in the ground. Whenever possible, however, the jays avoid valley oaks because acorn woodpeckers (Melanerpes formicivorus) defend them vigorously and thus attack the foraging jays. Therefore, the seed production by oaks that compete with valley oaks, in this case California blue oaks (Q. douglasii) results in increased predation of acorns by acorn woodpeckers - birds that store their acorns in so-called granary trees which provide no chance for germination or seedling establishment. All together, the blue oaks are thus indirectly competing with valley oaks, a phenomenon termed apparent predation, via the jays and the woodpeckers.
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