Forest inventory attributes predict the occurrence of cavities suitable for nesting by Wood Ducks (Aix sponsa)

Abstract

Large secondary cavity-nesting birds such as waterfowl and raptors rely on tree cavities developed principally through decay and damage processes or excavation by woodpeckers. Forest and wildlife managers are tasked with maintaining and producing these essential habitat components through forest management practices. Predicting where tree cavities are most likely to have developed based on commonly collected forest-inventory data would therefore be useful to better manage important bird species. Wood Ducks (Aix sponsa) are a common and well-studied secondary cavity-nesting species, though until recently, management of nesting habitat has primarily focused on installing artificial nesting structures as opposed to influencing forest-management practices. We measured and inspected 7869 trees and 1186 potential cavities in forests of north-central Minnesota during 2016–2018 to determine their suitability for use by nesting Wood Ducks. Two hundred and twenty-three of the cavities inspected (19%) were classified as suitable for use by nesting Wood Ducks. We fitted logistic regression models using tree- and stand-level forest attributes to predict the probability of an individual tree having at least one suitable cavity. The top model predicted well whether trees had suitable cavities (average area under the received-operated-characteristic curve value of 0.85 [95% CI; 0.82–0.87]) and had an additive relationship among diameter at breast height (DBH), tree health status, and tree species. We also found some support for including an interaction between DBH and health status. For every one-cm increase in DBH, the odds that a given tree would have a suitable cavity increased by 7% (95% CI; 6–9%). Dead and declining trees were more likely to have suitable cavities than healthy trees, with 834% (483–1420%) and 477% (276–807%) higher odds, respectively. In a comparison of seven common deciduous species with cavities, sugar maple (Acer saccharum) and American basswood (Tilia americana) were most likely to have developed cavities. These results can be applied to existing forest-inventory datasets to predict the availability of suitable cavities in the landscape and to improve forest management strategies that benefit Wood Ducks and other large-bodied secondary cavity-nesting species.