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).