Date of Award
Program or Major
Master of Science
Adrienne I Kovach
David M Burdick
Kate M O'Brien
Tidal-marsh birds that nest on the marsh surface are faced with numerous reproductive challenges. Most recently, the impact of rising sea-levels threatens to reduce or eliminate reproduction in tidal-marsh nesting birds. One species most vulnerable to sea-level rise is the saltmarsh sparrow (Ammodramus caudacutus), an obligate tidal-marsh breeding bird with reproduction that is strongly linked to the tidal cycle. The saltmarsh sparrow is a species of high conservation priority in the northeast United States (USDI 2008) and is globally vulnerable to extinction within the next 50 years (Bayard and Elphick 2011, Wiest et al. 2016). I combined molecular, behavioral, and ecological techniques to better understand adaptations in female saltmarsh sparrow nesting ecology. Using nest data collected at four New England marshes from 2011-2015, I investigated adaptations in nesting behavior of females with respect to nest placement and offspring sex ratio manipulation. I also conducted a management experiment to test the feasibility of artificial habitats to provide flood-free nesting habitat in the face of sea-level rise.
In Chapter 1, I sought to determine if structural nest characteristics differed by nest fate. I also investigated patterns of female nest site selection, using females with multiple nesting attempts throughout the study period, to determine if female saltmarsh sparrows modify their nesting behaviors based on prior experience. I found that nest characteristics differed by nest fate with fledged and predated nests built higher in the vegetation and in higher elevation areas of the marsh than those that flooded. Successful nests also had greater canopy cover and a lower proportion of high marsh vegetation than those that were flooded or predated. Nest height and elevation also differed between consecutive nesting events, consistent with a response to previous experience. Females whose first nesting attempt failed due to flooding constructed subsequent nests higher in the vegetation and in areas of higher elevation than those that were successful in their previous attempt. Females whose nests were predated in their first nesting attempt moved their second nests farther than females whose first nesting attempts were successful. I also found evidence for strong nest placement fidelity, as 84.5% of females renested between years within a distance smaller than the average core home range area (77m). These results demonstrate female saltmarsh sparrows exhibit plasticity in nesting behavior by changing structural nest characteristics following a cause-specific nest failure, which may be important for balancing selective pressures in the dynamic salt marsh environment. However, the exhibited plasticity may be insufficient to maintain reproductive success in the face of increased flooding predicted with sea-level rise.
Chapter 2 investigated if female saltmarsh sparrows are able to manipulate offspring sex ratios in response to environmental, temporal, or physiological conditions. The harsh environmental conditions of nesting in salt marshes provide a context for offspring sex manipulation to favor the sex with the greatest chance of survival or reproduction. I tested a priori hypotheses about the influence of environmental, temporal, and maternal effects on patterns of sex allocation at both the site and population level using sets of generalized linear mixed models. Across years and sites, I found an even offspring sex ratio of 1.03:1, with an alternating pattern of interannual variation between male and female bias at both the population and site level. Offspring sex ratios did not vary as a function of timing within the breeding season or in relation to tidal flooding, and was also independent of female condition at time of nest initiation. I also observed considerable within brood variation in offspring sex ratios with a higher degree of variation than expected under a normal distribution. Our finding of a 1:1 offspring sex ratio and interannual variation in a wild bird population is more consistent with the predictions of Fisher (1930) than those of Trivers and Willard (1973).
In Chapter 3, I tested the efficacy of artificial habitat islands for maintaining flood-free high marsh nesting habitat for saltmarsh sparrows. I created and installed four 4 ft. x 8 ft. floating island rafts, vegetated with Spartina patens and Spartina alterniflora in a marsh pool on Rachel Carson National Wildlife Refuge in Wells, Maine. Islands were monitored through the breeding season and winter. The islands have remained free of tidal inundation and supported vegetation growth and expansion, suggesting that floating habitat islands hold promise as a method for mitigating nest flooding in tidal-marsh-nesting birds. Changes in vegetation, loss of nesting habitat, and increased tidal inundation will reduce, if not eliminate, the reproductive ability of marsh-nesting birds, including the saltmarsh sparrow. Conservation actions are needed in the very near-term to identify solutions to mitigate nest flooding and maintain breeding populations until habitat is created in the longer term by natural marsh processes or habitat restoration efforts. Creation of artificial habitat islands offers such a potential short-term management action.
Benvenuti, Bri, "NESTING ADAPTATIONS AND CONSERVATION OF A THREATENED TIDAL MARSH-NESTING BIRD THE SALTMARSH SPARROW" (2016). Master's Theses and Capstones. 1093.