Date of Award

Fall 2018

Project Type


Program or Major

Natural Resources

Degree Name

Master of Science

First Advisor

Adrienne I Kovach

Second Advisor

Brian J Olsen

Third Advisor

Jeffery Foster


Hybrid zones can provide an understanding of the genetic basis of biodiversity maintenance and as well as insight into how interacting species respond to climate change, and how climate change may alter patterns of introgression. This body of research focuses on dynamics of hybridization between the Saltmarsh (Ammospiza caudacutus) and Nelson’s Sparrow (A. nelsoni) across two populations in the center of the hybrid zone to gain a window into both the evolutionary processes underlying the relationship between these species and the role of climate change and adaptive introgression on the future persistence of the two sparrows. In Chapter 1, I determined patterns of introgression between Saltmarsh and Nelson’s Sparrows on a fine-scale across a habitat gradient and on a broad-scale through comparison with known patterns in the southern range of the zone. I explored the fitness consequences of hybridization to female Saltmarsh, Nelson’s and hybrid sparrows in relation to environmental conditions and tidal marsh nesting adaptations in Chapter 2. Finally, in Chapter 3, I evaluated the relative fitness of male Saltmarsh, Nelson’s and hybrid individuals in relation to competitive ability and male condition. I intensively sampled sparrow adults (n = 218) and chicks (n = 326) and determined the success of 201 nests over two years at two marshes in the center of the hybrid zone located at Popham Beach State Park and Wharton Point on Maquoit Bay on the northeastern coast of the United States, between Brunswick, Maine and Phippsburg, Maine. I used a ddRAD sequencing approach to identify a panel of135 fixed SNPs, which I used to calculate a hybrid index and determine the genotypic composition of individuals and the level of admixture of the populations. In addition, a separate panel of 589 SNPs was used to assign paternity to offspring and reconstruct mating pairs. I compared genotypic composition and patterns of introgression across two sites in the center of the hybrid zone with previous work done in the southern portion of the hybrid zone. I tested for reduced survival of hybrid females in support of Haldane’s Rule and also for assortative mating between the species. I modeled daily nest survival and fledging success between Saltmarsh, Nelson’s and hybrid females in relation to tidal cycles and known tidal marsh nesting adaptations. Lastly, I compared the number of offspring sired by Saltmarsh, Nelson’s and hybrid males in relation to male condition, as measured by three secondary and one primary male sexual traits. I found that population density differences across the hybrid zone influenced patterns of introgression, such that in the center of the zone there is relatively equal backcrossing in both the Saltmarsh and Nelson’s Sparrow direction compared to asymmetric backcrossing toward the Saltmarsh Sparrow in the southern hybrid zone (Walsh et al., 2015a). Local site-specific characteristics of the two study populations influenced the distribution of genotypes and patterns of introgression across a tidal marsh habitat gradient, such that there were a higher number of hybrids and more backcrossing towards Nelson’s Sparrow at the inland than coastal site. I also observed twice as many recent-generation hybrid female nestlings than adults in the population, supporting Haldane’s Rule, and a significant correlation between mother and father hybrid index (r = 0.73, P <0.0001), indicative of assortative mating. I found differential fitness among Saltmarsh, Nelson’s and hybrid females. Birds with predominantly Saltmarsh Sparrow alleles had higher reproductive success than birds with predominantly Nelson’s Sparrows alleles, with hybrids being intermediate between the two. Fledging success models suggested that the number of offspring fledged also increased with two known tidal marsh nesting adaptations: nest height and nesting synchrony with tidal cycles. I found a positive relationship between hybrid index and fitness in daily nest survival in 2016, but not across both breeding seasons (2016 & 2017) combined, likely due to differing levels of nest flooding. The strongest and most consistent predictors of daily nest survival were nesting synchrony with lunar tidal flooding cycles (female behavioral adaptation) and daily maximum tide height. I also found differential male fitness, with Saltmarsh Sparrows siring more offspring than Nelson’s Sparrows (ANOVA; F = 3.81, P =0.04) and hybrids intermediate in fitness, although more similar to Nelson’s Sparrows. Cloacal Protuberance (CP) volume and body mass were significant predictors of interspecific fitness, providing evidence that pre and post copulatory sexual selection may be acting on body size and CP volume (as a proxy for sperm competition) to drive mating patterns within and between the Saltmarsh and Nelson’s Sparrows. Saltmarsh-Nelson’s Sparrow hybrid zone structure and maintenance appear to be driven by endogenous and exogenous factors at multiple spatial scales. Fitness differences among parental species and hybrids, relative population densities and species distributions, differential adaptation to local environments, and pre-zygotic and post-zygotic reproductive isolating mechanisms all play a role in the dynamics of this hybrid zone.