Evaluating the Effectiveness of Local and Landscape Attributes on Wood Thrush Nest Success in Contiguous Forests

Evaluating the Effectiveness of Local and Landscape Attributes on Wood Thrush Nest Success in Contiguous Forests

 

ABSTRACT

Eastern forest breeding birds have undergone steady and precipitous population declines over the last 50 years, and among them the Wood Thrush (Hylocichla mustelina) has exhibited some of the most serious population reductions. The causes of their population declines have largely been attributed to the fragmentation, degradation, and the complete loss of available habitat on both their breeding and wintering grounds, with losses of breeding habitat being 3 to 6 times more important than comparable habitat losses on their wintering grounds. Previous studies on Wood Thrush nest success in fragmented areas have shown that their nest success is higher with increasing forest cover in the surrounding landscape. Despite the implied significance of largely forested landscapes to Wood Thrush and other interior forest breeding birds, coupled with increasing fragmentation rendering extant contiguous forests increasingly important, patterns of Wood Thrush nest success in largely forested landscapes are not well studied and these patterns are relatively unknown. Therefore, the goal of this study was to identify what are the primary factors influencing the nest success and productivity of Wood Thrush in contiguous forests to identify characteristics of optimal nesting habitat. We were interested in the influence of patterns in land cover, edge metrics, habitat variables, and the invertebrate prey biomass available to them.

To determine the primary drivers of nest success and productivity, we nest searched and monitored the fate of 248 Wood Thrush nests at seven field site locations in Central Pennsylvania during the breeding seasons of 2018-2020. We quantified the percentage of different land cover types (forest, agriculture, and developed) at 0.5 km, 1 km, and 2.5 km scales around each nest, the distance to edge and edge types, and the vegetation around each nest site location, and we quantified the overall invertebrate and calcium-rich invertebrate biomass at each nest monitored during the 2020 breeding season. We also quantified the percent of habitat classified as timber harvests 0.5 km, 1 km, and 2 km around the nest site. To model nest success, we used models in Program MARK which allow for the inclusion of nest specific covariates and created model suites for each spatial scale.

We found that the Wood Thrush nest survival probability (NSP) in our study (0.3613) was higher than estimates reported in fragmented landscapes, but even within the largely forested landscapes in our study (2.5 km mean forest cover of 84.22 ± 10.99), the pressures from development and urbanization are what are most influential on Wood Thrush nest success. In addition, we found that forest structural and habitat characteristics around the nest are insignificant predictors of nest success. Our best fitting models at all three spatial scales (0.5 km, 1 km, and 2.5 km) included percent developed cover as a predictor variable, all having a negative relationship between the amount of developed cover and Wood Thrush nest success at each scale. At the 0.5 km and 1 km scale, we found that Wood Thrush NSP declined from 0.452 to 0.275 and 0.488 to 0.251 respectively over a developed cover range of 0% to 7%. Most significantly, we found that Wood Thrush NSP declined precipitously from 0.618 to 0.237 over a small range of 0% to 6% developed cover (equivalent to 0 to 117 hectares) within 2.5 km of a nest.

We found that timber harvests can potentially have a negative effect on Wood Thrush nest success when located nearby (1 km scale), but at a larger (2 km) and smaller scale (0.5 km), there was no negative influence. Wood Thrush NSP declined from 0.506 to 0.295 over a timber harvest cover range of 0% to 12% at the 1 km scale. Conversely, NSP was unaffected at the 2 km scale, with NSP estimates of 0.426 at 0% cover and 0.441% at 9% cover. Our results indicate that in moderate amounts, timber harvests in largely forested landscapes do not have significant impacts on Wood Thrush nest success, building confidence in a potential “tradeoff” between the early-successional and mature forest species and indicating that management for these two habitat types is likely compatible.

We did not find any significant relationships with macroinvertebrate prey biomass in the territory around a nest, and found that it was an insignificant predictor of Wood Thrush nest success, clutch size, and proportion of a clutch that successfully fledges. The mean non-snail macroinvertebrate biomass across all nests was 0.3883 g/m² (range 0.0155- 2.5187 g/m²), with site means ranging from 0.3518 g/m² (SGL 92) to 0.4787 g/m² (SGL 166). Prey biomass has been shown to drive territory selection and space use, and Wood Thrush adults in contiguous forests appear to nest only where there is enough invertebrate prey for successful nesting.

Our results demonstrate that even within primarily forested landscapes, human development and suburban sprawl in the surrounding landscape are still the primary drivers of Wood Thrush nest success, with their negative effects being both detectable and significant over 2 km into adjacent contiguous forest areas. Identification and management application of the factors influencing Wood Thrush nest success in high priority areas, contiguous forests, is necessary for the mitigation and reversal of their recent negative population declines. Management for this species on the breeding grounds should focus on minimizing development in areas surrounding large priority forest blocks, as well as consider that even seemingly small increases in development even within largely forested landscapes can have significant negative effects on the nest success of mature forest birds in the surrounding landscape.

Evaluating the Effectiveness of Local and Landscape Attributes on Wood Thrush Nest Success in Contiguous Forests

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