Plasticity in nesting adaptations of a tidal marsh endemic bird

Abstract

If individuals can perceive and manage risks, they may alter their behaviors based on prior experience. This expectation may apply to nest site selection of breeding birds, for which adaptive behavioral responses may enhance fitness. Birds that nest in tidal marshes have adapted to the challenges posed primarily by periodic, monthly tidal flooding and secondarily by predation. We investigated adaptive responses in nesting behavior of the saltmarsh sparrow (Ammospiza caudacutus), an obligate tidal‐marsh‐breeding bird, using 536 nests monitored across 5 years. Using linear mixed effects models, we tested whether nest characteristics differed among nests that were successful, depredated, or flooded, and we investigated whether females made changes in nest structure and placement according to outcome of their previous nesting attempt. Nest characteristics differed among females with different nest fates. Fledged and depredated nests were built higher in the vegetation and in higher elevation areas of the marsh than those that flooded. Successful nests had more canopy cover and were comprised of a lower proportion of high marsh vegetation (Spartina patens) than those that were flooded or depredated. Females with nests that failed due to flooding constructed subsequent nests higher in the vegetation and at higher elevation than those that were successful in their prior attempt, consistent with a response to previous experience. Eighty‐five percent of females renested within the average home range core area distance (77 m), indicating a high degree of nest placement fidelity. Females for which nests were depredated in their prior nesting attempt renested at a greater distance than females for which the previous nesting attempts were successful. Our findings suggest saltmarsh sparrows exhibit plasticity in nesting behavior, which may be important for balancing selective pressures in a dynamic environment. This plasticity, however, is insufficient to enable them to adapt to the increased flooding predicted with sea‐level rise.

Patterns of introgression vary within an avian hybrid zone

“…Nesting success of females is linked closely with habitat characteristics and nest site selection in this system, with the strongest driver of success being nesting behaviors that mitigate tidal flooding [ 42 – 45 ]. Differential adaptation supports the prevalence of Saltmarsh Sparrow alleles on coastal sites, where Nelson’s Sparrow females have a fitness disadvantage relative to inland marshes, which have a dampened tidal regime [ 38 , 41 ].…”

“…Further, the coastal marsh is part of an expansive coastal marsh network, while the inland marsh is located in a small cove that is surrounded by mostly forest and field. Although both sites experience daily and monthly tidal inundation, tide heights tend to be dampened in inland marshes relative to coastal [ 45 ]. In addition, through water-level monitoring on the marsh over the two-year study period, we know flooding rates were lower at the inland site compared to the coastal site [ 56 ].…”

Nest initiation and flooding in response to season and semi-lunar spring tides in a ground-nesting shorebird

“…by providing inferior conditions for the hatchlings leading to increased chick mortality [14, 22] or via increased risk of nest predation [52–56]. Predation risk may therefore be traded off with the risk of flooding, as flood-safe nests that are further away from the shoreline or more elevated might be more vulnerable to predation [21, 23]. Indeed, Snowy Plovers nesting on artificial nest mounds were more likely to survive extreme weather events, yet it is unclear whether such nests were also more prone to predation [32].…”

“…For example, Oystercatchers Haematopus ostralegus chose seemingly more elevated nest sites after experiencing nest flooding but when tested formally it turned out that their nest site selection was not different from random [20]. In contrast, female Saltmarsh Sparrows Ammospiza caudacutus that experienced nest flooding built their replacement clutches more elevated in vegetation than expected by chance [21]. Fifth, parents may be able to reduce the exposure of their nests to spring tides by shortening the incubation period.…”

“…For example, some ground-nesting birds construct elevated nest mounts or select elevated nest sites that won’t be reached by spring tides during the breeding season [10, 15, 17, 18, 20]. Such elevated nests, however, may make nests more conspicuous to predators [17, 20, 21]. Alternatively, parents may select nest sites far away from the highest tide line.…”