Date of Award

Spring 5-10-2025

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Ecology and Environmental Sciences

First Committee Advisor

Brian Olsen

Second Committee Member

Kristina Cammen

Third Committee Member

Benjamin King

Additional Committee Members

Serita Frey

Suzanne Ishaq

Abstract

Ecogeographic rules describe phenotypic patterns along environmental gradients that are observed across taxa. These trends can reveal shared adaptive responses to commonly experienced selection pressures. Many long-recognized ecogeographic rules have been described qualitatively yet have mixed empirical support, as pinpointing the underlying driver(s) of trait variation and its influence on fitness can be complex. Saltmarsh melanism, an ecogeographic pattern that describes increased melanization of integument in tidal saltmarsh vertebrates as compared to their interior relatives, is one such phenotypic trend that is not fully understood. The Ammospiza sparrows, three taxa that occur along North America’s Atlantic coast, exhibit saltmarsh melanism in correspondence with their degree of specialization to the habitat. Two selection pressures have been proposed as potential drivers of saltmarsh melanism: camouflage, as darker integument against gray saltmarsh sediments may obscure sparrows from aerial predators; and protection against keratin-degrading microorganisms, as melanin has anti-microbial and feather-strengthening properties, and keratin-degrading microorganisms may occur in high abundances in saltmarshes. Neither of these two hypotheses have been studied specifically in the Ammospiza sparrows.

Here, I investigated potential causes of saltmarsh melanism within these taxa. To do so, I expanded my investigation to drivers of melanized plumage more broadly—UV exposure, thermoregulation, camouflage, and protection against keratin-degrading microorganisms. Research approaches included 1) metabarcoding of bacterial and fungal taxa inhabiting Ammospiza plumage, which revealed low abundances of keratin-degrading microorganisms in feathers, 2) phylogenetic analyses of covariates of feather melanism across North American avifauna, which identified thermoregulation as a potential driver of melanism, 3) linear modelling that demonstrated decreases of feather melanism with temperature in male Saltmarsh and Seaside Sparrows, both of which display more specific adaptations to saltmarshes than do Nelson’s Sparrows, and 4) linear modelling that did not provide evidence for a contemporary influence of feather melanism on female reproductive success. Overall, my research points towards thermoregulation as a driver of feather melanism across Ammospiza populations and broadly across North American birds. Camouflage and defense against bacteria received little support for their role in influencing across-population color variation in Ammospiza. Further, sex-specific patterns in feather melanism suggest that sexual dichromatism may warrant further study for its role in saltmarsh melanism.

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