Data from: Kirtland's warbler occupancy and plantation density

Early studies into the habitat preferences of the Kirtland’s warbler (KW; Setophaga kirtlandii) suggested that these birds exhibited a preference for areas with high jack pine stem densities; therefore, jack pine plantations established as part of the KW recovery and conservation programs have been planted using a 1.5 x 1.8 m spacing (3,588 trees ha-1). In contrast, traditional pine plantations in the Lake States established for roundwood production are typically planted on a 2.1 x 2.4 m spacing that equates to 1,922 trees ha-1. Over more than 40 years, tree spacing in KW habitat plantations has gone largely unchanged, and until very recently there has never been an attempt to verify that these tighter spacings actually provide better KW habitat than would a traditional forestry spacing. We used a retrospective approach to assess the impacts of tree density on KW occupancy and observed an unexpected negative relationship between realized plantation density and maximum occupancy by KW singing males. This finding should be interpreted with caution due to the limited nature of this study; however, a lack of a positive relationship is entirely plausible given the narrow range of densities encountered in plantations, as well as the fact that uniform spacing of plantations should allow for the achievement of optimal jack pine cover at lower densities than would be required in a natural-origin stand.