Date of Award
Doctor of Philosophy
Joseph Clark, David Buckley, Arnold Saxton
Partners in Flight (PIF) recommends using silviculture to improve breeding habitat conditions for migrant landbirds. Alternative thinning treatments may benefit priority landbird species by increasing structural complexity in second-growth forests. However, the effects of thinning on landbird populations in oak (Quercus spp.)-hickory (Carya spp.) forests have not been experimentally demonstrated. I used a randomized and replicated large-scale manipulative experiment to evaluate the effects of thinning (i.e., crown-release and gap creation) on forest habitat characteristics and avian populations at the Tennessee National Wildlife Refuge. I collected data during 2001 (pre-treatment) and from 2002 to 2005 (1 to 4 years post-treatment) in 20-ha thinned (n = 8) and control (n = 4) plots. Using mixed model ANOVA with covariates, I compared habitat attributes, tree regeneration, avian population densities, daily nest survival rates, realized brood sizes (# fledged per successful nest), rates of brown-headed cowbird (Molothrus ater) brood parasitism, and nest-site characteristics between treatments. In addition, I used Program MARK to evaluate the influence of habitat factors at multiple spatial scales on predation rates of Acadian flycatcher (Empidonax virescens) and wood thrush (Hylocichla mustelina) nests.
Forest habitat attributes, avian population densities, and nest survival rates did not differ between control and thinned plots prior to treatment, indicating my experimental design (including the random allocation of treatments to plots, blocking, and the interspersion of plots across the study area) was sufficient for detecting treatment effects. Thinning resulted in a 29% difference in basal area between treatments (thinned = 20.3 m2 ha-1; control = 28.5 m2 ha-1). Compared to controls, thinned plots had significantly less overstory cover and midstory cover and significantly more downed wood and herbaceous and woody vegetation in the lower forest strata. Specifically, I detected greater densities of oak (Quercus spp.), yellow-poplar (Liriodendron tulipifera), and sourwood (Oxydenrum arboretum) saplings, and greater cover in poison ivy (Toxicodendron radicans) and blackberry (Rubus spp.) in thinned than control plots.
I used spot-mapping to estimate the densities of PIF priority species. Thinning had positive effects on the densities of seven species (eastern towhee [Pipilo erythropthalmus], eastern-wood pewee [Contopus virens], indigo bunting [Passerina cyanea], Kentucky warbler [Oporornis formosus], white-eyed vireo [Vireo griseus], yellow-breasted chat [Icteria virens], and yellow-throated vireo [Vireo flavifrons]), inconclusive or negligible effects on the densities of two species (Louisiana waterthrush [Seiurus motacilla] and worm-eating warbler [Helmitheros vermivorus]), and negative effects on the densities of two species (Acadian flycatcher and wood thrush).
I monitored 1,149 nests of 28 species. Predation accounted for 80% of all nest failures. Mayfield-adjusted nest daily survival rates of all species combined did not significantly differ between treatments. For all species combined, rates of cowbird parasitism varied annually but did not significantly differ between thinned (20.8%, SE = 2.3) and control (18.5%, SE = 3.7) plots. I assigned bird species to functional groups for further analyses. PIF priority mature-forest species exhibited nest daily survival rates (0.972 vs. 0.969), realized brood sizes (2.8 vs. 2.6), and parasitism rates (16.9 vs. 10.4%) that were comparable between thinned and control plots. Based on 162 nests in thinned plots, PIF shrubland species had nest daily survival rates of 0.958, realized brood sizes of 2.9, and parasitism rates of 13.6%; this functional group nested too rarely in control plots for analysis. Treatment effects were significant for the overstory and midstory nesting functional groups. Overstory nesters exhibited nest daily survival rates that were greater in thinned (0.982) than control (0.963) plots. Midstory nesters experienced greater parasitism rates in thinned (30.0%) than control (17.9%) plots.
I evaluated nest-site selection and factors affecting nest predation rates using 132 Acadian flycatcher and 112 wood thrush nests. In thinned plots, both species selected nest sites with greater overstory and midstory cover than found at random. I found little evidence that nest predation rates were influenced by the amount of agriculture in the local (314 ha) landscape or by distance to anthropogenic edge, perhaps because the landscape was predominantly forested (agriculture < 4%) and most nests were >350 m from an edge. In thinned plots, predation rates on wood thrush nests decreased with increasing overstory cover and increasing basal area in large trees; predation rates increased with increasing basal area in small-diameter trees. None of the habitat predictors I measured had a strong relationship to Acadian flycatcher nest predation rates in thinned or control plots. Model-averaged nest survival estimates for wood thrushes were 27.8% and 26.8% in thinned and control plots, respectively. Acadian flycatcher model-averaged nest survival estimates were 53.5% in thinned and 56.4% in control plots.
In summary, my results indicate that thinning had strong effects on forest habitat attributes and the demographics of some priority bird species. In the short term (1 to 4 years post-treatment), thinning appears to provide suitable breeding habitat for priority bird species that prefer dense understory vegetation or partially-opened overstories for nesting. Conversely, thinning had neutral or negative effects on some species and functional groups that nest in midstory vegetation, indicating there may be an ecological cost, in the short-term, associated with implementing this treatment. This treatment likely will have differential costs and benefits for avian populations as forest habitat conditions continue responding via successional dynamics and vegetative growth to the initial thinning operation.
Thatcher, Benjamin Stephen, "Evaluation of Forest Management to Improve Breeding Habitat for Songbirds in Oak-hickory Forests at Tennessee National Wildlife Refuge. " PhD diss., University of Tennessee, 2007.