Breeding Site Faithfulness, Reproductive Biology, and Adult Survivorship in an Isolated Population of Cassin's Finches

The Condor 87:494-510 0 The Cooper Ornithological Society 1985 BREEDING SITE FAITHFULNESS, REPRODUCTIVE BIOLOGY, AND ADULT SURVIVORSHIP IN AN ISOLATED POPULATION OF CASSIN’S FINCHES L. RICHARD MEWALDT AND JAMES R. KING ABSTRACT.-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 Moun- tain in southern Oregon. Cassin’s Finches (Curpoducuscussinii), usually consid- ered nomadic, returned each spring. Females were more faithful to breeding site than males. As evidenced by cloaca1 swelling and incubation patch formation, 96% of females laid eggsin the 18 days following 24 May. When June snow storms destroyed most nestsin 1975 and 1979, many females attempted secondnestings. Most apparently abandoned these second nestingsin 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 50150. 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 nest- site 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 Kernales. Cassin’s Finch (Curpoducuscussinii) inhabits eastern Oregon varied little in size from year open conifer woodlands of the interior moun- to year and that individuals showed high tains of western North America, generally breeding site faithfulness. This stimulated us nestingabove an elevation of 1,500 m. It breeds to examine and report here several related as- from southern interior British Columbia, pects of the summer biology and demography Montana, and northern Wyoming south to in- of this relatively little-known species.Our data terior southern California, northern Arizona, on Cassin’s Finches were obtained as a sec- and northern New Mexico. The finch is at least ondary result of a more extensive, seven-year an altitudinal migrant in parts of its range, and survey of year-to-year variation in the breed- migrates latitudinally as well. Wintering flocks ing avifauna of Hart Mountain, conducted by have been reported in the lowland valleys as systematic capture-recapture of birds in mist far north as southern British Columbia. Lati- nets. Our analysis is, therefore, retrospective tudinal migrants have been recorded regularly and confined to questions that can be exam- as far south as northern Baja California and ined with the aid of data obtained by such the Mexican highlandsto Durango, Zacatecas, methods. and Coahuila (AOU 1983). A limited literature (summarized by Salt STUDY AREA 1958, Orr 1968, and Samson 1976a) conveys Hart Mountain, a part of Hart Mountain Na- the impression that Cassin’s Finch is an op- tional Wildlife Refuge, is a typical block-fault portunistic species, characterized by a labile basalt formation in the Great Basin country of annual cycle, weak faithfulnessto breedingsite, southeastern Oregon. Elevations range from and geared to exploit variable sourcesof food. the 1,36 5-m lake surfacesin the Warner Valley The bird sharesthese characteristics with many to 2,444 m at Warner Peak. An escarpment other species of cardueline finches (Newton of nearly 1,000 m faces westward. The Hart 1972). In the only systematic studies of the Mountain population of Cassin’s Finches cen- summer biology of the Cassin’s Finch done ters on the isolated grove of ponderosa pines, thus far, Samson(1976a, b) found large annual henceforth referred to as “Blue Sky,” which is fluctuations in breeding populations of north- associatedwith riparian habitat on the com- ern Utah and regardedthe speciesas nomadic. paratively gentle eastern slope of the moun- In contrast, we found that a population of tain. Occupying a total area of about 136 ha, Cassin’s Finches on Hart Mountain in south- Blue Sky (Fig. 1) extends from about 1,830 to 14941 SITE FAITHFULNESS IN CA&SIN’S FINCH 495 FIGURE 1. Aerial view of the entire Blue Sky relict grove of ponderosapines on the easterlyslope of Hart Mountain, taken 07:45 on 10 June 1982. Warner Peak (2,444 m) is in the right center background.The Y-shaped Camp Creek study area extends along the left edge of the photograph. Post Meadow is in the left center foreground.To the right of and below Warner Peak are the headwatersof Guano Creek, which then flows to exit the photographin the lower left comer. On 10 June, the aspensat Blue Sky and upper Guano Creek were not yet leafed-out. 1,950 m elevation. Three small permanent The ponderosa pine grove at Blue Sky was spring-fed creekswith their associatedriparian severely reduced, if not nearly demolished, in strips, including several small meadows, en- the fall and winter of 1866-1867, when the compassabout 30 ha between the lateral edges 11th Cavalry Regiment of the United States of the tree canopy. At Blue Sky, the riparian Army established Camp Warner on the site habitat is dominated by ponderosa pines, and cut trees for a stockade, shelters,and fire- quaking aspens(Populus tremuloides), willows wood. Camp Warner was abandoned in the (Salix scouleriana), and alders (Alnus tenui- early spring of 1867. filia). Of the greater study area, 106 ha is char- Representative core samples taken from acterized by sagebrush(Artemisia tridentata), among the 5 14 ponderosa pines counted along rabbit brush (Chrysothamnus nauseosus),bit- Camp Creek in 1974 show that all but four terbrush (Purshia tridentata), grasses,forbs, giants, each more than 250 years old, were no and scattered western juniper (Juniperus oc- more than 100 years old. Examination of the cidentalis), continuous with the surrounding additional 25 ha of the Blue Sky pine grove shrubsteppe. showsthe samepattern; that is, a few old giants, We centered our study on Camp Creek (Fig. but mostly trees of the same age as on Camp l), the southernmost of three permanent, Creek. Given the rate of growth of these pines spring-fed streams that originate in and flow and the predilection of Cassin’s Finches for through Blue Sky. The Camp Creek springs, nesting above 15 m in tall pines (Orr 1968, at about 1,950 m elevation, flow 1,300 m to Samson 1976a), we believe that the present join Guano Creek at 1,830 m elevation. The population of Cassin’s Finches probably es- Y-shaped riparian habitat of Camp Creek in- tablished itself, or more likely re-established cludes 5.4 hectareswithin the lateral edgesof itself, no more than 50 to 70 years ago. the canopy of usually mixed pines and aspens. During our periods of residence, variously We made systematic studies during the sum- April to October, we recorded weather data mers of 1973-l 979 and gatheredsupplemental near our camp site (1,885 m elevation) on data in summers of 1972, 198 1, 1982, and Camp Creek with a hygrothermograph, a py- 1984. ranograph, and a rain gauge. We also have in 496 L. RICHARD MEWALDT AND JAMES R. KING 101 Annual Mean 63.8cm Twice (1975 and 1979) during the study (1973-l 979), stormsin mid-June brought cold rain and substantial snow. Because in 1975 (17-l 9 June and 24-25 June) and againin 1979 (16-l 8 June) these storms lasted 36-48 hours, reproductive failure was nearly universal among species that were incubating eggs or with young in the nest at Blue Sky and higher on the mountain. Aspen leaves of early clones on Camp Creek came from bud to full size in the period from JFMAM idJJASOND 20 May to 3 June. Later clonesmoved through the same sequences10 to 15 days later. We noted as much as 15-days difference in phe- 161 Annual Mean 5.I”C nology among years, with an apparent relation to variable persistence of snow cover in the aspen groves. The production of forbs and grassesvaried conspicuouslyamong years and was especially luxuriant in 1974. METHODS To maximize captures, we established 64 net sites,eight to 10 of which we used each day in rotation (Fig. 3). Net sitesaccommodated from - one to four black nylon 36-mm mesh, four- JFMAM J J A s 0 ILN D panel mist nets, 12 m long. Most nets were FIGURE 2. Estimated mean monthly precipitation and placed perpendicular to stream flow, and two temperature on mid-Camp Creek, from records taken at or more nets at a site were placed end to end. Refugeheadquarters 1940-l 979 (precipitation) and 197l- Nets were set with the bottom trammel line 1980 (air temperature), and from our records taken at about 0.15 m above the ground and the top Camp Creek during our tenure there (1973-1979). 2.15 m high. The 64 sites accommodated 94 nets. Nets, open from dawn to usually about hand long-term records (National Oceanic and 1 l:OO, were emptied hourly, the birds were Atmospheric Administration) of air temper- banded and measured, and then were released ature and precipitation that were taken by U.S. near the site of capture. Data recorded at each Fish and Wildlife Service personnelat the Hart capture and recapture included site, hour, age, Mountain Refuge headquarters (1,7 13 m ele- sex, wing-length (first capture each season), vation) on the open shrubsteppeplain 15 km weight, reproductive condition, and pertinent NNE of Blue Sky. Our simultaneous records notes on plumage and molt. Overall capture permitted us to estimate the annual patterns efficiency of Cassin’s Finches was 59 per 1,000 of temperature and precipitation at Blue Sky net hours (Table l), but typically exceeded 100 by correlation with the records from Refuge per 1,000 net hours on most days in late May headquarters.

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