Cavity-excavating birds, such as woodpeckers, are ecosystem engineers and are often assumed to rely upon wood decay fungi to assist in softening the wood of potential excavation sites. Endangered red-cockaded woodpeckers (Picoides borealis; RCWs) are the only birds known to solely excavate through the sapwood and into the heartwood of living pine trees and generally take many years to complete their excavations. These birds may have developed a partnership with wood-inhabiting fungi to facilitate the excavation process. Past attempts to understand the complex relationships between cavity excavators and fungi relied on visual surveys of fruiting bodies, or evidence of decay, resulting in a one bird, one fungus paradigm. Using molecular methods, I investigated the relationships between RCWs and fungi, and found that the relationships between cavity-excavators and fungi involve multiple fungal species and are far more complex than previously imagined. Through a field survey, I showed that RCW excavations contain distinct communities of fungi, and propose two hypotheses to explain this result, (1) RCWs select trees with distinct fungal communities (tree selection hypothesis), or (2) RCWs promote distinct fungal communities via their excavations (bird facilitation hypothesis). By swabbing the birds, I found that RCWs carry fungal communities similar to those found in their completed excavations, demonstrating that RCWs may directly facilitate fungal dispersal during the excavation process. Through a test of the bird introduction hypothesis which implemented human-made experimental drilled cavity starts (incomplete excavations), half of which were inaccessible to the birds, I showed that RCW accessibility influences fungal community development in excavations. This experimental evidence demonstrates that the relationship between RCWs and fungal communities is a multipartite symbiosis may be mutualistic. Finally, by tracking fungal community development in experimental cavity starts through time, I also demonstrated that the fungal communities found in RCW excavations undergo succession, and that this process is influenced by the birds. The relationships described in this body of work provide the basis for future studies on cavity excavators and fungi, and also have implications for a diverse community of secondary cavity nesters, wood-inhabiting fungi, forest ecology, and the conservation of biodiversity. / Ph. D.