4. DISCUSSION
Decades of field observations in a range of bird species suggest the importance of insects to birds during the breeding season, as protein demands are increased while producing eggs and provisioning nestlings (Capinera, 2011 ,Vitz & Rodewald, 2012). We identify correlations between food resources and GFWO nest site location and home range size, along with nest cavity characteristics that facilitate successful broods and reveal the importance of abandoned woodpecker cavities for secondary cavity nesting birds. Additionally, our results suggest a novel trade-off between excavating live trees versus dead/decaying trees, evident in the differences in nest success between natural cavities and abandoned woodpecker cavities.
Resource driven site locationAll recorded orders of insects collected within our study were found at all occupied and unoccupied site types, though not every insect order was found at each sweep netting location, nor at every visit. Previous literature has indicated that Coleoptera and Hymenoptera are in high proportions of woodpecker diets (Beckwith & Bull, 1985; Hess & James, 1998; Fayt, Machmer, & Steeger, 2005; Pechacek & Kristin, 2010), and as we predicted in our first objective, the biomass of both of these insect orders were higher around GFWO nests than unoccupied sites and increases in their biomass corresponded with decreased GFWO home ranges, up to 15,000 m2. In addition, we found similar relationships between Orthoptera and GFWO sites and home ranges.
Our findings indicate that resource availability (e.g. insect biomass) may be driving the location and home range sizes of this ecosystem engineer, as GFWO nests were located in areas that corresponded with insect availability, and home ranges shrank in correlation with increases in those same insect orders. This is in accordance with previous literature which indicates that woodpeckers reduce their defended areas when resources were abundant, and chose nesting sites based on resource availability (Pasinelli, 2000; Tingley, Wilkerson, Bond, Howell, & Siegel., 2014). The differences we found in insect biomass between occupied and unoccupied sites were most likely due to fine scale variation in vegetation and water availability indistinguishable by our vegetation associations (Huang, Zhao, & von Gadow, 2015).