The Effects of Thinning on Songbird Abundance and Diversity in the New Jersey Pine Barrens
University of Delaware
Many wildlife species associated with disturbance-dependent habitats have been declining due to suppression of forest management techniques, such as thinning and prescribed fires. One species specifically, the prairie warbler (Setophaga discolor), relies on the early-successional shrubland habitat that is potentially enhanced by these forestry management techniques. The objective of this study was to predict breeding-season habitat occupancy of prairie warblers within the New Jersey Pine Barrens on unmanaged lands as compared to private and state lands that have been purposely thinned and or managed with fire. One hundred and fifty counts were conducted at 18 study areas, 9 managed (75 point counts) and 9 unmanaged (75 point counts), from 11 June-3 August, 2012. Vegetation characteristics were measured at point counts including percent and type of ground cover, forest basal area, vertical vegetation profile, and canopy coverage. We used program PRESENCE to build occupancy models to explain prairie warbler presence in the Pine Barrens, and evaluated the models using Akaike Information Criterion. Out of the 13 detection models only time of day model had ΔAICc ≤ 2 and this variable was used in further occupancy models assessing habitat characteristics. Of the 36 a priori variable habitat occupancy models that were assessed, only two had ΔAICc ≤ 2 (Table 2). These were model averaged and accounted for 53.5% of the AICc weight. Therefore the top predictive model for prairie warbler occupancy = - 2.391(canopy coverage) –3.039(% grass) + 1.460(% understory at 2m) + 1.487(% baresoil). For sites where prairie warblers were detected I found that percent understory coverage at 2m was higher and percent canopy coverage and grassland were lower. Bare soil had a negligible effect due to large unconditional variance. Our results suggest that to increase prairie warbler occupancy management techniques should focus on providing more early successional shrubland habitat. Since prairie warblers are considered a key avian indicator of ecosystem health and management effectiveness the New Jersey pinelands should provide additional patches of these early successional habitats to protect prairie warbler populations.