Phenotypic variation within species is an important area of evolutionary biology, for it is this variation which the process of selection directly acts upon. However, the biological factors that contribute to the generation and maintenance of this variability are still not well understood. Part of the ambiguity arises from the occurrence of multiple layers of natural and social selection factors which may be occurring simultaneously, and at different hierarchical levels within and between populations. How these factors interact with each other, and at what level, can generate highly variable traits in which the frequency of diversification differs from the individual to the species level. In addition, in conjunction with selective factors, stochastic genetic variation can impact the evolution and maintenance of phenotypic traits in ways that we do not fully understand, dependent on features such as population size, mutation rate, and the dispersal ability of individuals.
Plumage polymorphism, where multiple forms of a trait are maintained within a species, is a widespread occurrence in many families of birds, but is especially common in birds of prey. The high prevalence of variable plumage color and pattern makes raptors an excellent system in which to study the factors that may contribute to high phenotypic diversity in a species. A large number of polymorphic birds of prey have been identified and examined, but evidence for how multiple selection and stochastic factors interact to maintain these traits has not been well-studied. To explore this question, my research focused on the occurrence and overlap of natural and social selection factors, and how they may interact with genetic stochasticity, in the American Kestrel (Falco sparverius). The American Kestrel is an ideal subject for this study, for it represents one of the most dichromatic raptors with a range of plumage variation found across the New World. In order to investigate the evolutionary factors that originate and maintain this range of plumage variation, I integrated behavioral data collected within California, with museum studies, demographic data, and an analysis of the genetic structure and clinal variation of plumage from across North America.
The American Kestrel has been known to have highly variable plumage, but little work has been conducted to examine specific plumage traits in detail. Part of the difficulty in performing this task lies in the extensive range covered by the species. Found from the highest latitudes of North America to the southern tip of South America, the American Kestrel is almost continuous in its distribution across the New World. In my first chapter I use a combination of museum specimens from multiple collections to examine the full range of the variation found in one highly variable plumage trait of male American Kestrels, the black and white coloration on the tail. By doing so I was able to identify three repeatable morphs and four levels of variation in the color of the tail for males. In my first and subsequent chapters I then use this newly defined polymorphism to examine how the variation present in this trait relates to selective and stochastic factors at both the individual and population level.
Selection through social interactions, such as mate choice and competitive interactions, has often been associated with the evolution of plumage traits in birds. Using both a captive population of American Kestrels maintained in Maryland and free-living individuals from study sites in Northern California, my first chapter examines both the heritability and differences in reproductive fitness associated with the polymorphic tail color of male kestrels. I found that the tail morph appears to change little between molt for adults, and both the morph and level of variation are heritable in the captive population. However, there appeared to be little difference in the reproductive fitness of males with different morphs or variation level in the free-living groups. Males with larger amounts of black and white were shown to nest earlier, but this result was only recovered at one of four study sites. Overall, my research detected no consistent sign of selection on the black and white polymorphic tail of American Kestrels through social interaction for breeding birds.
A lack of evidence for the use of the tail color in behavioral interactions does not mean that the trait is not under other forms of selection. A wide-ranging species could encounter a large scope of different microhabitats and ecological factors. Natural selection, such as specialization in habitat type or differing fitness under divergent climatic variables, could potentially drive the maintenance of multiple morphs if different types were specialized for different ecological areas. Such a pattern is often thought to result in clinal distribution of forms across large areas. In my fourth chapter, I used plumage data from 633 male kestrels collected during the breeding season from across North America to construct a map of the frequencies of the three morphs and four variation levels for the continent. A geographic cline was discovered, where birds along the eastern Atlantic Coast showed higher frequencies of both the pattern and level of variation with the greatest degree of black and white. Extending to the west and ending at the Pacific Coast, birds showed a much higher frequency of morphs and patterns with smaller amounts of the two colors. The geographic division in morph frequencies suggested that broad scale climatic or ecological variables might play a role in the distribution of American Kestrels with different tail morphs. However, no correlation was found between a range of bioclimatic variables and tail color. There was also found to be no relationship between morph and variation level, and broad habitat type. Thus, it appears that the highly variable tail pattern of male American Kestrels is not strongly affected by either natural or social selection factors.
The inability to discover any strong relationship between selection factors and the polymorphic tail color of American Kestrels suggests that the trait may be under weak or no selection at this point in time. In such cases, the major influences on the tail color may instead be from stochastic genetic variation, and changes in population size, genetic population structure, and the movement of birds across the landscape. In chapter three, I used demographic and genetic data from a long-term study program of American Kestrels in Saskatchewan, Canada to show that kestrels do experience rapid changes in population size. Findings also suggested that the species appears to be very fluid with its movement into local breeding areas, and is readily affected by changes in land-use and habitat. In chapter four, I turn back to the continent wide data set and compared the different morph frequencies seen with population structure constructed from 375 genetic samples of American Kestrels collected during the breeding season. The genetic structure detected separated out three populations, one in the southeast corresponding with the subspecies F. s. paulus, one along the Atlantic Coast, and a third that appeared to encompass the rest of the continent. This does not correspond exactly with the geographic cline of morph frequencies, but both the genetic and plumage data highlight that the Atlantic Coast is a unique area for the American Kestrel.
Broadly, my dissertation used a combination of research at both the individual and population level to identify a new polymorphic plumage trait in American Kestrels and to determine that the trait appears to exist in different frequencies among populations on the North American continent. While no direct association was found between tail color and natural selection factors at the broad geographical level, or social selection factors at the individual level, the findings do not rule out these areas completely. Overall, the work conducted here suggests that the factors influencing plumage polymorphism in birds of prey are complex, and that stochastic genetic variation may have a larger impact on the maintenance of plumage polymorphism then originally recognized.