What factors limit bird distributions? After all, they can fly, and sometimes it seems like perfectly good habitat is available but unoccupied (at present) by a species. Nick Mason and I investigated the drivers of distributional limits in the Acorn Woodpecker in South America. This clown-like Woodpecker is found only in the forests of the Colombian Andes. Surprisingly, it has never been recorded from nearby Ecuador. We first tested the possibility that the Colombian Andes have a special climate that is not present in Ecuador, but did not find any support for this idea. Instead, we found that Acorn Woodpecker distribution is closely tied to the presence of Colombian Oaks; this is evidence that Acorn Woodpecker distribution in the Northern Andes is limited by a tree, not by climate. Foraging data for Acorn Woodpeckers in Colombia is scarce, but it is most likely that they use Colombian oaks as a food resource. However, Acorn Woodpeckers in Colombia seldom consume acorns and do not store them in granary trees, as Acorn Woodpeckers in North America do. Instead, preliminary evidence suggests that Acorn Woodpeckers in Colombia may feast on the sap of Colombian oak trees. It seems likely that other birds have specialized relationships with particular plants that are food resources, and Nick and I hypothesize that similar specializations limit the distributions of other tropical birds.