Breeding site faithfulness, reproductive biology, and adult survivorship in an isolated population of cassin's finches

We used mist nets in a capture-recapture demographic study of the birds of an isolated relict grove of ponderosa pines (Blue Sky) on Hart Mountain in southern Oregon. Cassin's Finches (Carpodacus cassinii), usually considered nomadic, returned each spring. Females were more faithful to breeding site than males. As evidenced by cloacal swelling and incubation patch formation, 96% of females laid eggs in the 18 days following 24 May. When June snow storms destroyed most nests in 1975 and 1979, many females attempted second nestings: Most apparently abandoned these second nestings in early July when the finches left Blue Sky to wander more widely over Hart Mountain while they underwent their annual molts. Most yearling females and males were sexually active and apparently nested. Although obscured by reduced mobility of females during incubation and early brooding, the sex ratio was close to 50/50. There were 20 to 25 pairs per hectare of suitable (pine) finch habitat. Breeding density may be limited by the zone around the female that is defended by the male during nestsite selection and nest building. The minimum annual survival rate for finches one year old and older was 0.64 (SE = ? 0.02) for males and 0.60 (SE = ? 0.03) for females. Cassin's Finch (Carpodacus cassinii) inhabits open conifer woodlands of the interior mountains of western North America, generally nesting above an elevation of 1,500 m. It breeds from southern interior British Columbia, Montana, and northern Wyoming south to interior southern California, northern Arizona, and northern New Mexico. The finch is at least an altitudinal migrant in parts of its range, and migrates latitudinally as well. Wintering flocks have been reported in the lowland valleys as far north as southern British Columbia. Latitudinal migrants have been recorded regularly as far south as northern Baja California and the Mexican highlands to Durango, Zacatecas, and Coahuila (AOU 1983). A limited literature (summarized by Salt 1958, Orr 1968, and Samson 1976a) conveys the impression that Cassin's Finch is an opportunistic species, characterized by a labile annual cycle, weak faithfulness to breeding site, and geared to exploit variable sources of food. The bird shares these characteristics with many other species of cardueline finches (Newton 1972). In the only systematic studies of the summer biology of the Cassin's Finch done thus far, Samson (1976a, b) found large annual fluctuations in breeding populations of northern Utah and regarded the species as nomadic. In contrast, we found that a population of Cassin's Finches on Hart Mountain in southeastern Oregon varied little in size from year t year and that individuals showed high breeding site faithfulness. This stimulated us to examine and report here several related aspects of the summer biology and demography of this relatively little-known species. Our data on Cassin's Finches were obtained as a secondary result of a more extensive, seven-year survey of year-to-year variation in the breeding avifauna of Hart Mountain, conducted by systematic capture-recapture of birds in mist nets. Our analysis is, therefore, retrospective and confined to questions that can be examined with the aid of data obtained by such methods.