Environmental Influence on the Ontogeny, Allometry, and Behavior of the Song Sparrow (Melospiza melodia)

DOCTORATE DISSERTATION: …This thesis explores the contribution of the environment to the Song Sparrow pattern of geographic variation. In chapter 1, via the analysis of reciprocal transplant experiments, I address what processes are acting to produce differentiation in Song Sparrow morphology. By comparing the ontogeny of young reared in their native environment to that of young reared in an ecologically differentiated environment, I demonstrate that the bill morphology is particularly plastic, whereas the limbs are not. In addition, it is the body size that is environmentally flexible whereas the growth rate (allometric slope) is relatively invariant. Therefore, environmental variation alone can explain some of the morphological diversity among Song Sparrow populations. This study and others have revealed a lack of concordance in phenotypic plasticity among traits within an organism. The mechanism for this discord has not been described. In chapter 2, I investigate the factors which determine the level of phenotypic plasticity exhibited by specific traits. A multivariate analysis of size and shape of Song Sparrow populations found along the Pacific coast shows that shape variation conforms to geographic barriers and thus may reflect the pattern in which bill characteristics are positively allometric and all other traits are not supports the conclusion that body size is an environmentally plastic trait; morphological characteristics exhibit levels of phenotypic plasticity in proportion to their rate of growth with respect to body size. Because of allometry these traits grow at different rates. Consequently, a change in body size can change an entire suite of traits. Thus local physical environmental factors which alter body size can create striking patterns of morphological diversity among Song Sparrow populations. Based upon behavioral observations the Song Sparrow is believed monogamous. However, molecular analyses often illustrate that social pairings do not reflect patterns of gene flow. Mixed reproductive strategies abound within monogamous social systems. Several avian taxa exhibit geographic variation in mating system. Given the importance of environmental variation to Song Sparrow morphological variation, one might expect behavioral mechanisms to vary also. In chapter 3, I assess the level of behavioral plasticity in two ecologically differentiated populations of Song Sparrow. The rate of extra-pair fertilization can be influenced by the opportunity for the sexes to meet and copulate discretely. Consequently, I predicted that extra-pair fertilizations would be more common in a visually occluded, dense population. However, DNA-fingerprinting reveals no evidence of extra-pair fertilization in either population. Likewise there is no difference in level of parental care or sperm competition in the two populations. These results argue that despite morphological flexibility in relation to local environmental factors, the Song Sparrow is truly monogamous. It is proposed that this strict monogamy is maintained by the constraints of a single pair rearing two broods in one season. Phenotypes result from a complex nexus of interactions between the genes and the environment. The Song Sparrow is morphologically highly variable. A proportion of this variability is environmentally flexible-transplanted young deviated toward the phenotype of the foster population. Consequently, geographic patterns of morphological diversity may not be a direct representation of underlying additive genetic variation. Rather, geographic variation represents the norm of reaction and phenotypic plasticity itself may be the object of selection. In this way, avian phenotypic plasticity may provide alternative morphologies in response to variable selection pressures and thus create patterns of evolutionary diversity.