Acorn harvesting by acorn woodpeckers: annual variation and comparison with genetic estimates

Questions: (1) How does the effort that birds invest in harvesting acorns, including the distance the acorns are moved – a key factor affecting population structure of the trees – covary with the size of the acorn crop? (2) How well are harvest patterns, previously inferred by indirect, genetic methods, matched by data from direct observations of harvesting? Organisms: The acorn woodpecker (Melanerpes formicivorus), a cooperatively breeding species that is highly dependent on acorns, which are stored in specialized storage trees known as granaries. Methods: We observed acorn harvesting over 4 years at Hastings Reservation in central coastal California, a period over which acorn crops varied considerably in size. Results: Birds harvested 94% of acorns from a small number of trees located within 150 m of their granary. The distances travelled by birds to harvest acorns and the number of trees from which acorns were harvested were both greater in a poor acorn year than when the crop was good. Birds did not necessarily prefer the species of acorn that was most abundant. The distance birds travelled to harvest acorns, harvesting overlap among groups, and the number of trees from which acorns were harvested generally matched the findings of Grivet et al. (2005), indicating that an indirect genetic approach can be effective when direct observation of seed dispersal is difficult.