Interconnected nesting success
In our second and third objectives, we predicted that the same cavity metrics that influenced GFWO nest success would also influence SCB, and that SCB would have higher nest success in abandoned woodpecker cavities. As predicted, all SCB had higher nest success rates in abandoned woodpecker cavities than in natural cavities and cavity type was the strongest predictor for all species, with the BEWR having the least impact, followed by the BCTI, and largest influence on ATBC. Additionally, GFWO had higher success in trees with lower decay and higher vegetation cover, which was mirrored in SCB; BCTI and ATBC were more likely to produce fledglings in trees with low decay, and BEWR were more likely to produce fledglings in cavities with high vegetation cover. The BEWR was the only species not impacted by decay, potentially explained by its generalistic nesting behavior (Taylor, 2003). We observed successful BEWR nests built in metal pipes or direct sun, thus experiencing wide temperature swings throughout the day, indicating that unstable nesting environments may be a deterrent for other cavity nesting birds, but not this species.
Also in line with our third objective, we predicted that abandoned woodpecker cavities would share characteristics making them better nesting cavities than natural ones. To this, SCB within our study had higher success rates within abandoned woodpecker cavities (81-93%), than in natural cavities (41-56%). The structure of abandoned woodpecker cavities present on our sites were distinctly different from their natural counterparts; on average they were significantly higher in trees, of lower decay, smaller DBH, and increased vegetation cover, all characteristics that protect eggs and fledglings from shifting internal temperatures and predation (Copeyon, 1990; Ojeda, Suarez, & Kitzberger, 2007;Pakkala et al., 2019). Considering that SCB are reliant on pre-existing cavities to create their nests, the factors that drive the creation and design of woodpecker cavities may then dictate the success of local SCB.