L. Richard Mewaldt

SONG DIALECTS AS BARRIERS TO DISPERSAL IN WHITE-CROWNED SPARROWS, ZONOTRICHIA LEUCOPHRYS NUTTALLI

Studies of avian communication systems have revealed an impressive number of cases in which a bird species exhibits a mosaic pattern of clusters of males singing a song or songs peculiar to each cluster (the most intensive study is that of Baptista, 1975). In some species, these subgroups are discrete, but in other species the differences between subgroups gradually grade from one to another. The geographic dimensions of these so-called dialects may be quite local, a few km 2 (Marler and Tamura, 1962), or more regional, a few hundred km 2 (Nottebohm, 1969). No unifying theory exists to explain the diversity of dialect systems, but several workers have suggested that mating probabilities might be influenced by song dialects with the effect of reduced genetic exchange among dialect groups (Nottebohn, 1969; Baptista, 1975). In the few species carefully studied, it has been found that the song dialect is learned early in ontogeny (Marler and Tamura, 1964; Mundinger, 1975). The dialect phenomenon suggests two kinds of questions. First, how do song dialects originate? One view of the historical origin of a dialect system is analogous to a sympatric speciation model in which dialects arise within a continuously distributed population with the passage of time (Nottebohm, 1969). An alternative proposal, essentially an allopatric model, is that dialects arise from a colonization or

Seasonal Sex and Age Ratios in Populations of the White-Crowned Sparrows of the Race Gambelii

The existence of geographic and temporal segregation of the sexes and age classes during the migration of birds has been an accepted concept for many years. For relatively few species, however, are the data adequate to demonstrate convincingly that there are differences with respect to age or sex in the schedule of migration, in the routes of migration, or in wintering populations (Eaton, 1933; Frieling, 1934; Nice, 1937; Thomson, 1939; van Dobben and Miirzer Bruyns, 1939; Lack, 1943, 1944; Deelder, 1949; Bellrose, Scott, Hawkins, and Low, 1961; Jehl, 1963; Schifferli, 1963). The available data are not sufficient to reveal common or consistent patterns of differential migration in any group of birds, if, indeed, such patterns exist. The evolution of this opinion has been ably reviewed by Tordoff and Mengel (1956), and the history of the subject need not be reiterated here. We believe that a rigorous analysis of patterns of differential migration must be based on a larger fund of comparative data than is currently available. The present report is intended in part as a contribution to this fund based on data on the White-crowned Sparrow of the race Zonotrichia leucophrys gambelii. It should be emphasized also that differential migration has a general relevance to many types of investigations in avian biology, quite apart from its basic relationship to problems of population dynamics. The existence of differential timing, routes, and destinations in the migration of different components of a population has an obvious bearing, for instance, on investigations of population genetics, speciation, homing and navigation, and physiological ecology. Thus, intensive study of differential migration is especially pertinent in taxa such as Zonotrichia that have been widely used in investigations in avian biology.

The Annual Cycle of White-Crowned Sparrows (Zonotrichia leucophrys nuttalli) in Coastal California

The basic unit of temporal organization in long-lived animals is the annual cycle, In birds this includes most prominently the periods of winter maintenance, reproduction, molt, and migration or dispersal. To attain maximal, or at least adequate, fitness, all of these essential processes must occur and be timed in individuals and populations so that conflicting demands for time and energy are minimized and they must exploit as well as possible the cyclic variations of the environment. The structure of the annual cycle thus represents evolutionary tendencies toward optimization, including, for instance, the length of the reproductive period, the pattern and duration of molt, and the proclivity for migration. Although much can be learned about the adaptive basis of the annual cycle by intensive investigation of individual taxa or geographic populations, much more can be learned by comparisons of taxonomic or geographic variants. Through nearly four decades of research, beginning with the pioneering investigations by Blanchard (summarized by DeWolfe 196S), the annual cycles of the crowned sparrows, Zonotrichia spp., have been analyzed especially thoroughly in this context. It is paradoxical that certain aspects of the annual cycle and general biology are least well known in the form of Zonotrichia Zeucophrys that is most readily accessible from urban and academic centers. 2. 1. nuttalli, hereafter called Nuttall’ s Sparrow, is the only nonmigratory form of Zonotrichia in North America, and is thus of particular interest in a comparative context. Nuttall’ s Sparrow inhabits the narrow coastal fog zone of California between about 34” and 40” N latitude. We report below the results of a five-year investigation of the cycles of reproduction and molt in Nuttall’ s Sparrows on Point Reyes, near the middle of their latitudinal range.

Demography of White-Crowned Sparrows (Zonotrichia Leucophrys Nuttalli)

Analysis of a White-crowned Sparrow population in coastal California resulted in es- timates of the magnitude of variation in longevity (a) among cohorts in the same breeding season, (b) between sexes, and (c) between years. Mean longevity calculated from early juvenile life averaged ;13 mo whereas mean longevity calculated from the egg stage was =6 mo. Survival of fledglings to 1 January was generally lower and more variable among cohorts than survival beyond January as adults. Reproductive information from a sample of known-age females indicated that clutch size and number of nestlings increased with female age. Age-specific longevity and clutch size were combined to calculate the net reproductive rates of two female cohorts. These were 1.042 and 1.025, respec- tively. A combination of survival information from five cohorts and information on the numbers of males and females in reproductive condition suggested an effective sex ratio of one male per 1.2 females. This departure from strict monogamy reduces the genetically effective population size by 10% .

The use of space by white-crowned sparrows: Juvenile and adult ranging patterns and home range versus body size comparisons in an avian granivore community

Summary1.Ranging behavior in a nonmigratory population of white-crowned sparrows Zonotrichia leucophrys nuttalli residing in the coastal scrub habitat of central California was studied by trapping and recapture. Over 12,000 captures of more than 1,700 individuals were analyzed. In their first breeding season, young Z. l. nuttalli produced from early broods bred near their own hatching site, but those from later broods bred farther away (Fig. 3).2.As adults, females had more restricted movements in summer than did males. In winter, females expanded their ranging movements more than males. We hypothesized that higher vagility of females in winter is related to their being socially subordinate to males in flocks. This causes them to wander among different flocks or to live a solitary existence.3.Employing a bivariate statistical model we found the most satisfactory description of home range size to be given by a 50% equal frequency ellipse. By this method, the average Z. l. nuttali home range was estimated to be 6–7 ha (Fig. 6).4.The home range of four other passerines in the same trap system has been estimated previously. We compared our results on Z. l. nuttalli with reported results on the four other passerines to examine the relationship between home range size and body size among granivores. The best-fit line has the equation: home range (ha)=0.044 W(g)1.31 (Fig. 7). The slope of this line is much steeper than that published for the relationship of mammalian home range and body size and is slightly steeper than indicated by previous work on birds. Our slope is also steeper than that of the relationship between standard metabolic rate and body size for passerines but is similar to a theoretically derived regression coefficient describing the power required for flight in relation to body size. We hypothesized that in birds the energetic demands of flight have influenced the evolution of home range size.