Evaluating outcomes of management targeting the recovery of a migratory songbird of conservation concern

Abstract

Background: Assessing outcomes of habitat management is critical for informing and adapting conservation plans. From 2013-2019, a multi-stage management initiative, led by the American Bird Conservancy (ABC), aims to create >25,000 ha of shrubland and early-successional vegetation to benefit Golden-winged Warblers (Vermivora chrysoptera) in managed forested landscapes of the western Great Lakes region. We studied a dense breeding population of Golden-winged Warblers at Rice Lake National Wildlife Refuge (NWR) in Minnesota, USA, where ABC initiative management was implemented to benefit the species.

Methods: We monitored abundance before (2011-2014) and after (2015-2016) management, and we estimated full-season productivity (i.e., young recruited into the fall population) from predictive, spatially explicit models, informed by nest and fledgling survival data collected at sites in the western Great Lakes region, including Rice Lake NWR, during 2011 and 2012. Then, using biologically informed models of bird response to observed and predicted vegetation succession, we estimated the cumulative change in population recruitment over various scenarios of vegetation succession and demographic response.

Results: We observed an 32% decline in abundance of breeding pairs and estimated a 27% decline in per-pair full-season productivity following management, compared to no change in a nearby control site. In models that ranged from highly optimistic to progressively more realistic scenarios, we estimated a net loss of 72-460 juvenile Golden-winged Warblers produced from the managed site in the 10-20 years following management. Even if our well-informed and locally validated productivity models produced erroneous estimates and the management resulted in only a temporary reduction in abundance (i.e., no change in productivity), our forecast models still predicted a net loss of 137-260 juvenile Golden-winged Warblers from the managed area over the same time frame.

Conclusions: Our study site represents only a small portion of a massive management initiative; however, the management at our site was conducted in accordance with the initiative's management plans, the resulting vegetation structure is consistent with that of other areas managed under the initiative, and those responsible for the initiative have described the management at our study site as successful Golden-winged Warbler management. Our assessment demonstrates that, at least for the only site for which pre- and post-management data on Golden-winged Warblers exist, the ABC management initiative is having a substantial and likely enduring negative impact on the species it purports to benefit. We suggest that incorporating region-specific, empirical information about Golden-winged Warbler-habitat relations into habitat management efforts would increase the likelihood of a positive response by Golden-winged Warblers.

Keywords: Abundance; Full-season productivity; Locally informed conservation; Management outcomes; Songbirds; Vermivora chrysoptera.

Figure 1. Long-term population trends for Golden-winged Warblers by region and globally from the Breeding Bird Survey.

Trends in Golden-winged Warbler counts along routes sampled during the Breeding Bird Survey (Sauer et al., 2014) from 1966–2013. Modeled numbers of birds per BBS survey route (open circles) and 95% credible intervals are displayed for (A) the global population, (B) the Appalachian breeding distribution segment, which comprises <5% of the global population, and (C) the Minnesota breeding population, which comprises ∼50% of the global population. Urgent calls for Golden-winged Warbler conservation action often cite the global trend (A), which has stabilized in recent years because western populations remain stationary or increasing and the influence of the Appalachian population on global trends has decreased as that population asymptotes toward zero.

Figure 2. The study site before and after management and a comparable area managed under the same prescription with the same result.

Vegetation at Rice Lake National Wildlife Refuge in Minnesota (A) before and (B) during the first growing season after management intended to benefit Golden-winged Warblers and associated young forest species and (C) a landscape described as managed for Golden-winged Warblers in Bald Eagle State Park in Pennsylvania (Bakermans, Ziegler & Larkin, 2015, J Larkin, ABC, pers. comm., 2015). Inset in A demonstrates the complexity in pre-management vegetation structure at our treatment study site. Image C demonstrates that management in Minnesota replicated vegetation structure of landscapes described as managed areas for Golden-winged Warblers by an ABC employee in the Appalachian region. Photos by H Streby (A and inset) and G Kramer (B and C).

Figure 3. Core territories of breeding pairs of Golden-winged Warblers before and after management.

Core breeding territories (white polygons) of Golden-winged Warbler pairs in our study sites at Rice Lake National Wildlife Refuge during 2013–2016. Core breeding territories in the treatment site are displayed for (A) 2013 (n = 62 pairs) and (B) 2014 (n = 62 pairs) before vegetation shearing, and (C) 2015 (n = 45 pairs) and (D) 2016 (n = 45 pairs), the 2 breeding seasons following vegetation shearing. Areas of vegetation shearing are identified by green polygons before management and red polygons after management. Core breeding territories in the control site are displayed for the same 4 years (E–F) with 19 pairs in each year. All core breeding territories were delineated based on spot-mapping, mist-netting, and observations of nest building, nestling feeding, and territorial behavior, are intended for census information, and do not represent total area used by birds for their song territories or home ranges (Streby, Loegering & Andersen, 2012). Base map from ESRI World Imagery.

Figure 4. Spatially explicit model of full-season productivity for Golden-winged Warblers breeding in the study area before and after management.

Productivity surfaces derived from spatially explicit full-season productivity models for Golden-winged Warblers breeding at our treatment site at Rice Lake National Wildlife Refuge, Minnesota, USA (A) before and (B) after management implemented with the intention of benefiting the species and related young forest-associated species. Non-nesting area (hatched) is comprised of roads, gravel parking areas, and grassland and productivity values from non-nesting areas are not included in total productivity estimates. Management increased the amount of non-nesting area and decreased productivity for the remaining nesting area from 1.45 fledglings raised to independence per breeding pair prior to management to 1.04 fledglings per pair immediately after management. Base map from ESRI World Imagery (both images are identical for ease of comparison of model outputs, thus the base maps are not intended to display location of vegetation shearing).