2.2 Nest Location and Monitoring
We used the East Foundation’s extensive long-term breeding bird dataset,
constructed over 6 years, to create a heat map of areas most likely to
contain nesting GFWO (Baumgardt et al., 2019). We then used the Point
Density tool in ArcGIS version 10.3 (Environmental Systems Research
Institute, Redlands, CA, USA) to take a 500 m² fishnet sample, and
interpolate density values across our study location. Within areas of
high GFWO density, we placed 12 1-km2 survey plots
(Figure S1) and from mid-April to late May, 2019 we visited each plot
four times using the spot mapping technique to locate nesting GFWO
(Martin & Geupel, 1993).
After locating GFWO nests, we searched 150 m2 grids
centered around each nest every 3-5 days between April and July 2019 to
document active SCB nests (Rodewald, 2004). To select GFWO unoccupied
sites, we placed 150m2 grids 300 m away from occupied
sites that had the same vegetation association but no observed GFWO
activity (sightings, calling, drilling, foraging, and nesting) and
searched for SCB nests in the same way. The vegetation associations were
determined by the East Foundation’s hierarchical vegetation
classification system, created in 2011-2012 where a vegetation
association was defined by the dominant and subdominant species
(Snelgrove, Dube, Skow & Engeling, 2013). To determine SCB nesting
tendencies and any differences in cavity metrics between abandoned
woodpecker cavities and natural cavities, we recorded and monitored all
empty cavities we found in each grid throughout the breeding season.
We monitored each SCB and GFWO nest every 2-5 days to determine nest
success; a nest was considered successful if ≥1 fledgling was observed
outside the nest. After fledging, we measured the following nest metrics
that have historically been predictors of cavity nesting success: the
height of the nest measured from the center of the cavity opening to the
base of the tree (height), the tree’s DBH, diameter of the cavity
opening (opening), the depth of the cavity (depth), and decay ranking
(decay), where a rank of one indicated a live tree and rank seven
indicated a dead tree with no branches, bark, and soft stem (Dobkin,
Pretare, & Pyle, 1995; Bonar, 2001; Cockle, Martin, & Wesolowski,
2011; Berl. Edwards, & Bolsinger, 2015). Because increased vegetation
cover may be detrimental for cavity nesting birds (Schaaf, 2020), we
used 0.5 x 0.5 m² cover boards to estimate the percentage of vegetation
cover at each cavity (Nudds, 1997; Chotprasertkoon, Pierce, Savini,
Round, Sankamethawee, & Gale, 2017).