Heterogeneity in breeding productivity is driven largely by factors affecting nestlings and young fledglings in an imperiled migratory passerine

Abstract

Identifying factors that drive variation in vital rates among populations is a prerequisite to understanding a species' population biology and, ultimately, to developing effective conservation strategies. This is especially true for imperiled species like the golden‐winged warbler (Vermivora chrysoptera) that exhibit strong spatial heterogeneity in demography and responds variably to conservation interventions. Habitat management actions recommended for breeding grounds conservation include timber harvest, shrub shearing, and prescribed fire that maintain or create early successional woody communities. Herein, we assessed variation in the survival of nests [n = 145] and fledglings [n = 134] at 17 regenerating timber harvest sites within two isolated populations in Pennsylvania that differed in productivity and response to habitat management. Although the overall survival of nests and fledglings was higher in the eastern population than the central population, this was only true when the nest phases and fledgling phases were considered wholly. Indeed, survival rates of nestlings and recently fledged young (1–5 days post‐fledging) were lower in the central population, whereas eggs and older fledglings (6–30 days post‐fledging) survived at comparable rates in both populations. Fledglings in the central population were smaller (10% lower weight) and begged twice as much as those in the eastern population, suggesting food limitation may contribute to lower survival rates. Fledgling survival in the central population, but not the eastern, also was a function of habitat features (understory vegetation density [positive] and distance to mature forest [negative]) and individual factors (begging effort [negative]). Our findings illustrate how identifying how survival varies across specific life stages can elucidate potential underlying demographic drivers, such as food resources in this case. In this way, our work underscores the importance of studying and decomposing stage‐specific demography in species of conservation concern.

Food availability aligns with contrasting demographics in populations of an at‐risk songbird

“…Importantly, we sampled each site twice, corresponding with two distinct phases of the annual breeding cycle: (1) when males arrived at breeding grounds and initiated territories, hereafter “first‐arrival” (eastern: 4–6 May, central: 6–9 May, Figure A1 ) and (2) when males and females both provision nestlings (Fiss et al., 2016 ; McNeil, 2019 ), hereafter “nestling‐rearing” (eastern: 5–7 June, central: 12–15 June, Figure A1 ). We selected these dates based on extensive prior work conducted within both subpopulations (Fiss et al., 2020 , 2021 ; McNeil et al., 2024 ; McNeil, Rodewald, Robinson, et al., 2020 ; McNeil, Rodewald, Ruiz‐Gutierrez, et al., 2020 ). Because both stages are known to occur earlier in the eastern subpopulation (McNeil, 2019 ), we always sampled this region first.…”

“…We acquired Golden‐winged Warbler fledgling body mass data from McNeil et al. ( 2024 ). We sampled fledglings for 2 years in each subpopulation (eastern: 2014–2015 and central: 2016–2017) and from 8 and 11 managed nesting sites in the eastern and central subpopulations, respectively (19 total; Figure A1 ).…”

“…A disparity in prey availability has been hypothesized to be a potential driver of starkly divergent Golden‐winged Warbler population trends between central Pennsylvania (hereafter “central subpopulation”) and eastern Pennsylvania (hereafter “eastern subpopulation” [McNeil et al., 2024 ]). Within managed Golden‐winged Warbler nesting habitat, the eastern subpopulation is documented to be more productive in many respects; this subpopulation is reported to have 10% greater clutch sizes, 9% heavier nestlings, 35% more fledglings produced per successful nest, 184% (~3×) greater full‐season productivity, 100% (2×) lower begging by fledglings, and 177% (~3×) greater likelihood of adult site occupancy than the central subpopulation (Larkin & Bakermans, 2012 ; McNeil et al., 2024 ; McNeil, Rodewald, Robinson, et al., 2020 ).…”

“…A disparity in prey availability has been hypothesized to be a potential driver of starkly divergent Golden‐winged Warbler population trends between central Pennsylvania (hereafter “central subpopulation”) and eastern Pennsylvania (hereafter “eastern subpopulation” [McNeil et al., 2024 ]). Within managed Golden‐winged Warbler nesting habitat, the eastern subpopulation is documented to be more productive in many respects; this subpopulation is reported to have 10% greater clutch sizes, 9% heavier nestlings, 35% more fledglings produced per successful nest, 184% (~3×) greater full‐season productivity, 100% (2×) lower begging by fledglings, and 177% (~3×) greater likelihood of adult site occupancy than the central subpopulation (Larkin & Bakermans, 2012 ; McNeil et al., 2024 ; McNeil, Rodewald, Robinson, et al., 2020 ). A food‐availability hypothesis has been suggested (McNeil et al., 2024 ) given that other factors have not been found to influence Golden‐winged Warbler reproductive success; such as, warblers from the central and eastern subpopulations overwinter together (Kramer et al., 2018 ) and vegetation structure did not predict warbler nest survival within managed Appalachian nesting sites (including Pennsylvania [McNeil et al., 2017 ]).…”

“…In this study, we tested the hypothesis that the central Golden‐winged Warbler subpopulation, where breeding output and responses to conservation action are weak, had lower food availability than the eastern subpopulation, where both patterns are reversed (McNeil et al., 2024 ; McNeil, Rodewald, Robinson, et al., 2020 ; McNeil, Rodewald, Ruiz‐Gutierrez, et al., 2020 ). To test this hypothesis, we compared primary prey availability (leaf‐roller caterpillars; family: Tortricidae) and physical indicators of food availability (plasma lipid metabolites, body mass from adult males, and fledgling body mass) between the two subpopulations.…”