Demographic Threats to a Trailing-edge Population: Survival, Productivity, and Climate Vulnerability of Virginia's Saltmarsh Sparrows (Ammospiza caudacuta)

Abstract

Climate change-induced sea level rise (SLR) is accelerating the loss of coastal marshes throughout the mid-Atlantic, increasing extinction risk for marsh-obligate species such as the Saltmarsh Sparrow (Ammospiza caudacuta). Saltmarsh Sparrow populations have been steadily declining since the late 1980s, and the species has exhibited range contraction at the southern extent of its breeding range in Virginia, USA. The majority of research on this species has focused on core and leading-edge populations in New England. However, trailing-edge populations, those near the receding edge of a shifting range, are predicted to be disproportionately vulnerable to climate-induced extinction and often operate under distinct demographic dynamics. At the trailing-edge of the Saltmarsh Sparrow breeding range, the factors limiting population growth may differ substantially from those documented elsewhere and could therefore require unique management strategies. To identify limiting factors to Saltmarsh Sparrow persistent at the trailing-edge and understand how SLR might affect these dynamics, we investigated three components influencing demographic threats to Saltmarsh Sparrows: 1) the prominent causes of nest failure and their relationship to nest-level characteristics; 2) the availability of suitable nesting habitat in marsh migration zones under projected SLR metrics; and 3) rates of apparent adult survival and adult sex ratios across the tailing-edge population. We monitored 113 nets across four marshes—two on the seaside and two one the bayside of Virginia's Eastern Shore peninsula—from May through July 2023-2025 using a combination of nest surveys, iButtons temperate dataloggers, and camera traps to determine nest fate and causes of nest failure. We used nest location and elevation data collected with a high-precision GPS to delineate suitable nesting habitat and applied The Nature Conservancy's marsh migration projections to estimate habitat availability and productivity under 3-ft and 6-ft SLR metrics. Lastly, we used mark-recapture data from 432 uniquely marked adult Saltmarsh Sparrows collected across two sites in 2021 and four sites in 2023-2025 in a robust Cormack-Jolly-Seber model to estimate apparent survival and characterize adult sex ratio. Nest monitoring revealed that predation was the leading cause of nesting failure, accounting for 64-90% of failures across all three years—contrasting with the leading-edge and core-range literature, which identifies flooding as primary threat to Saltmarsh Sparrow nests. Daily predation probabilities at our sites were 2-3 times higher than those reported from the next nearest population in New Jersey, and predation showed no relationship with measured nest-level characteristics, suggesting that predator dynamics rather than nest placement are driving reproductive failure. Cumulative nest survival across the 23-day nest cycling ranged from only 3.7-15.8%, well below the 20-48% reported from core and leading-edge populations. We also documented unexpectedly high rates of apparent nest abandonment (5-13% of failures), raising concerns about adult female mortality or mid-season dispersal. Under future SLR metrics, suitable nesting habitat in migrated marsh was substantially reduced: when habitat suitability was defined by distance to upland edge alone, projected productivity was approximately 46% of present-day levels in high productivity years. When elevation constraints were incorporated, suitable habitat and projected nest success declined by over 90%. Finally, our mark-recapture analyses revealed pronounced sex-biased apparent survival, with females exhibiting approximately 42.5% lower apparent survival than males (females: 0.23, 95% CI: 0.18-0.29; males: 0.40, 95% CI: 0.34-0.46) and a corrected adult sex ratio of approximately 3.5 males per female. Given that females provide all parental care, this demographic imbalance represents a potential bottleneck limiting reproductive output at the trialing-edge. The Virginia Saltmarsh Sparrow population faces compounding threats from elevated predation pressure, future habitat loss under SLR, and reduced female apparent survival—dynamics that diverge markedly from those documented at the core and leading-edge of the range. To protect against continued population declines, managers will need to consider targeted predator management strategies, protection and enhancement of existing high-quality nesting sites, and facilitation of marsh migration to offset habitat loss. Given the disproportionate conservation value of trailing-edge populations for maintaining species-wide adaptive capacity, coordinated management action at the Virginia trailing-edge is an urgent priority for the species' persistence.