Population Structure in Salt Marsh Song Sparrows: Part I. Environment and Annual Cycle

Abstract

The Song Sparrows (Melospiza melodia) that live on the salt marshes of San Francisco Bay, California, have attracted the attention of students of birds for the past sixty years. The principal reason for this has been their morphological distinctness from all other Song Sparrows, including those that live in areas close to the salt marshes. Such a pattern of differentiation and distribution has always raised the problem of how the salt-marsh populations maintain themselves as distinct entities. The study reported now is a continuation of previous work on salt-marsh Song Sparrows; the questions asked were, what characteristics of salt-marsh populations influence or have some relationship to the maintenance of their morphologic and/or genetic integrity, and how do these characteristics express themselves? Certain details bearing on these questions that are supplied in this and a succeeding paper have been interpreted in the light of the theoretical and factual framework partly supplied by such authors as Wright (1940; 1943), Mayr (1942; 1953), Miller (1947), and Marshall (1948). The Song Sparrows studied exist partly isolated from other populations of Song Sparrows on San Pablo salt marsh, Richmond, Contra Costa County. They offered advantages not otherwise generally available to the worker in population ecology of birds: they were abundant and were situated in a discrete unit of salt marsh; they belonged to a group genetically distinctive to the extent of being accorded the taxonomic status of race (M. m. samuelis); they were non-migratory; they had been studied from a general ecological viewpoint by Marshall (loc. cit.); and they belonged to a widespread species for which Nice (1937; 1943) had compiled a definitive life history based on studies in Ohio. Thus, most of the effort in studying the birds was directed to the investigation of population structure. Since these Song Sparrows differ in certain respects from those in Ohio, a number of features of their life history that bear on population structure are included. I have not offered a comparison of these differences except where it has seemed to be particularly significant. Generous help and counsel in the preparation of this report have been given me by Alden H. Miller, Frank A. Pitelka and Ray F. Smith; their suggestions have applied to all parts of the study. Responsibility for validity at all levels remains mine, however. The habitat photographs were taken by Joseph G. Hall. My wife, Lora Lee Johnston, assisted with preparation of manuscript. It is a pleasure to acknowledge the aid of these people.