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).