Habitat edges influence the distribution of nest predators for Seaside Sparrows, but not nest placement or success

Abstract

Nest failure for coastal marsh bird species is primarily caused by predation and nest flooding. As sea level rise makes nest flooding more likely, the threat of nest predation will constrain the potential adaptive responses of marsh nesting species. Thus, understanding the predictors of nest predation is important for the conservation of salt marsh-dwelling bird species, such as the Seaside Sparrow (Ammospiza maritima). Predator activity may be influenced by landscape features (particularly habitat edges), potentially making nest predation predictable. We aimed to understand the predictability of Seaside Sparrow nest predation relative to two major landscape features: roads and tidal rivers, as both of these edges may be entryways or attractants for predators in marshes. In coastal Georgia, USA, we assessed mammalian predator activity relative to the two features of interest, and hypothesized that mammalian predator activity would be greater close to roads and tidal rivers. We also recorded Seaside Sparrow nest locations and nest predation events and hypothesized that nest predation events would increase with increasing predator activity. Consistent with our first hypothesis, mammalian predator activity increased close to roads and tidal rivers, but mammalian predator distribution did not explain the spatial variation in Seaside Sparrow nest predation thus not supporting our second hypothesis. Seaside Sparrow also placed their nests in locations with high mammalian predator activity, indicating that the ability to avoid nesting in high-risk areas may be constrained by habitat or resource limitations. Additionally, mammals may not be the primary nest predators, as we found that one bird species—Marsh Wren (Cistothorus palustris)—contributed substantially to nest predation rates. Understanding the predictability of mammalian predator distribution can allow for focused predator management efforts, such as exclusion, to habitat edges where we found the highest mammalian predator activity, which could relax the constraint of nest predation on Seaside Sparrow’s ability to respond to the intensifying threat of sea level rise.

Interspecific interactions disrupted by roads

“…New habitats created by road corridors (see Section III.1.a) can act as ecological traps if animals that breed there experience higher predation or parasitism (Section III.4.a) rates. Road proximity is usually negatively related to bird nest survival in temperate regions (Falk, Nol & Burke, 2011;Yoo & Koper, 2017;Newsome & Hunter, 2022) as edge-associated predators use these new habitats for hunting (Small & Hunter, 1988;Thogmartin, 1999;Dijak & Thompson, 2000;Lahti, 2001;Fraser & Whitehead, 2005;Newmark & Stanley, 2011). By contrast, some studies have found no effect (Huhta, 1995;Bechet, Isenmann & Gaudin, 1998;Mettenbrink, Dreitz & Knopf, 2006;Svobodov a, Š alek & Albrecht, 2007) or a positive effect of road proximity on nest survival (Delgado García, Arévalo & Fern andez-Palacios, 2005;Angkaew et al, 2019;da Silva et al, 2019).…”