Age-dependent variability in the migratory orientation of the savannah sparrow (Passerculus sandwichensis)

Variability characterizes the orientation of migratory birds much as it does other aspects of their behavior. If one migrant behaves differently from three other migrants, the proper tactic is to find out why. What are the sources of variability? Experience, including individual responses to experience, is one obvious source of variability. Although individual differences in orientation behavior need not be a function of experience, they often are (see Graue 1965, Brown and Mewaldt 1968, Keeton and Gobert 1970, Keeton 1972, Wallraff 1972, W. Wiltschko and R. Wiltschko 1975, W. Wiltschko, R. Wiltschko, and Keeton 1976, Ralph 1978, Edrich and Keeton 1978, Williams 1978, Bingman 1981, R. Wiltschko and W. Wiltschko 1981, R. Wiltschko, Nohr, and W. Wiltschko 1981, Beacham 1982, Gauthreaux 1982). The effect of experience on orientation might manifest itself in several ways: more experienced individuals might (1) be able to select or maintain a direction based on less information (e.g. Keeton 1972, Wiltschko and Wiltschko 1975, Bingman 1981), (2) display greater flexibility in cue utilization (e.g., Wiltschko and Wiltschko 1975), (3) be able to switch more easily from one source of directional information to another (e.g. Baker and Mather 1982), or (4) be less susceptible to the distracting effect of the moon (Brown and Mewaldt 1968) or artificial horizon glows (Williams 1978, Beacham 1982, Gauthreaux 1982). In any case, inexperienced migrants are more likely to commit orientational errors (see Drury and Keith 1962; DeSante 1973, 1983; Ralph 1978, 1981; McLaren 1981). Greater individual variability among naive migrants would not be surprising in light of their lack of migratory experience and their maturational differences. On the one hand, naive migrants might display less consistent compass orientation on a nightto-night basis, which would yield greater within-individual variability. Greater within-individual variability would be indicative of less accurate orientation (see Emlen 1969, 1972; Moore 1978, in press). On the other hand, a sample of naive migrants might display a wider array of individual preferences and greater between-individual variability. Both withinand between-individual variability were analyzed in this study to test the hypothesis that naive migratory Savannah Sparrows (Passerculus sandwichensis) have greater individual variability in their compass orientation than do experienced migrants. Savannah Sparrows were mist netted in late August 1981 in eastern North Dakota near Grand Forks and transported to southcentral Mississippi, where orientation experiments were conducted. The age of the birds, either hatching-year (HY) or adult (AD), was determined upon capture by the extent of skull pneumatization. I housed the birds individually in an outdoor aviary under natural photoperiod and the local geomagnetic field (0.53 Gauss, mN = 3600, 640 inclination). Both HY and AD birds deposited subcutaneous fat and displayed nocturnal locomotor activity (Zugunruhe) before testing. Emlen funnel cages (Emlen and Emlen 1966) were used to record the orientation of migratory activity. Angular deviations were calculated according to Zar (1974). Orientation tests were conducted under clear, starry skies on moonless nights from 15 September through 30 October 1981 in an open field adjacent to the aviary. Birds were placed in orientation cages at 1900 (CST) with a clear view of the setting sun and were removed 3 h later. A bird's position within the arrangement of orientation funnels was randomly varied from night to night. Nights when naturally occurring fluctuations in the geomagnetic field exceeded a K-index value of 3 (>40 gamma) for the 3-h intervals during or preceeding a test night were not included in the analysis. [See Moore (in press) for additional details.] The mean heading from a night's cage activity is usually in a seasonally appropriate southerly direction whether or not the bird possesses migratory experience. When the nightly means are plotted for both experienced (AD) and naive (HY) birds, the means of the sample distributions are essentially the same (Figs. 1 and 2), although the HY distribution is characterized by greater scatter [compare measures of concentration for adult (r = 0.802) and naive birds (r = 0.618)]. Differences between the two groups of migrants appear when individual behavior is examined. Night-to-night consistency in orientation is especially evident among the birds with migratory experience (Table 1, Fig. 1). The high r values and low angular deviations associated with the distribution of mean headings for each adult point to narrow within-individual variability. Yet not all experienced adult migrants are oriented in the same southerly direction. Several AD birds consistently oriented their activity to the SSW-SW (AD 03, AD 11, AD 18, AD 22), whereas others (AD 08, AD 21) were just as well oriented to the SSE. This pattern is confirmed after the variability in adult headings is partitioned into betweenand within-group components by an analysis of variance. Variability among adult individuals accounts for a significant portion of the variability in headings (F = 2.32; df = 7,40; P < 0.05). The problem of applying an analysis of variance procedure to cir-