Multistate site occupancy estimation of white‐headed woodpeckers (Picoides albolarvatus) in managed forests

Background/Question/Methods

Structure retention is used to address biodiversity conservation objectives in managed
forests by providing current and future snag resources for wildlife. The practice is considered critical
to wildlife in intensively managed forests where timber production is the primary goal, but few studies
have demonstrated a clear link between stand-scale attributes of structure retention and metrics of
species habitat use, such as occupancy and reproduction. In this study, we used a Bayesian multistate
site occupancy model to understand how structure retention influences the probability of occupancy
and nesting by white-headed woodpeckers (Picoides albolarvatus) on industrial forests in California,
USA during the 2010 breeding season. Our objective was to estimate state probabilities (i.e.,
occupancy and nesting) for white-headed woodpeckers at the harvest-unit scale and identify
relationships between each state probability and habitat covariates that can be directly impacted by
forest management decisions, including snag density in the harvest unit and proportion of mature
forest on the landscape.

Results/Conclusions

Mean occupancy probability was estimated to be 0.98 (95% credible interval
[CRI]: 0.89-1.00) and the probability of nesting given occupancy was 0.93 (95% CRI: 0.75-0.99),
resulting in an unconditional nesting probability of 0.89 (95% CRI: 0.72-0.97). Snag density was
positively associated with nesting probability, while neither covariate was good at predicting
occupancy. The model indicated a negative effect of mature forest proportion on nesting probability,
though there was considerable uncertainty. Both detection of occupancy given nesting and detection
of nesting increased with survey period, while detection of occupancy at sites without nesting
decreased over time. The high occupancy and nesting probabilities in 2010 suggest that current
structure retention policies have provided the necessary habitat conditions for white-headed
woodpeckers nesting in harvest units of northern California. Harvest units with >2 snags ha^-1 (stem
diameters >25.4cm) had a probability of nesting that approached 1, suggesting a minimum threshold
of retention that could serve as the biological basis for management targets.