Patterns of growth (reviewed by Ricklefs 1968, 1969; O'Connor 1984) and the development of endothermy (reviewed by Dawson and Hudson 1970, Dunn 1975, Hill and Beaver 1982) have been well-studied in altricial wild birds, especially passerines. But few studies compare grown and thermogenesis in separate populations of the same species. Results of such studies with emberizids varied among species. King and Hubbard (1981), for example, found that nestlings from subarctic, subalpine, and low-altitude montane populations of White-crowned Sparrows (Zonotrichia leucophrys gambelii) grew at similar rates. In contrast, Rogers (1985) reported that the growth rates of nestlings in different populations of Savannah Sparrows (Passerculus sandwichensis) varied in response to the different environmental constraints of the localities in which they were reared. Nice (1937) and Smith et al. (1982) documented patterns of nestling growth in mainland (Ohio) and insular (Mandarte Island, British Columbia, Canada) populations of Song Sparrows (Melospiza melodia), respectively, and found that they were similar to those reported for most other passerines by Ricklefs (1968, 1969) and O'Connor (1984).

In 1985-1986, we had the opportunity to examine the growth of nestlings from a third race of Song Sparrows, M. m. micronyx, which is endemic to San Miguel Island near Santa Barbara, California. We also studied the development of endothermy in these young birds, a process not hitherto described for nestling Song Sparrows. We report both in this paper.