Nesting of the Sage Thrasher, Sage Sparrow, and Brewer's Sparrow in Southeastern Idaho

Abstract

-I examined the territory size, mating success, nest placement, nest development, and nesting success of the three passerine species restricted to the sagebrush (Artemisia tridentata) habitat in southeastern Idaho. Territories defended by male Sage Thrashers (Oreoscoptes montanus) were larger than those defended by either Sage Sparrows (Amphispiza belli) or Brewer's Sparrows (Spizella breweri). All but one of the territorial Sage Thrashers (n = 19) were successful in securing mates and nesting. Fifty-three percent of the territorial Sage Sparrows (n = 30) and only 23% of the displaying Brewer's Sparrows (n = 30) secured mates and nested. Thrashers nested either on the ground below sagebrush or in the branches of sagebrush plants. Brewer's and Sage sparrows nested only in the shrub canopy of sagebrush. Average incubation and nesting periods (rounded to the nearest whole day) for the Sage Thrasher, Sage Sparrow, and Brewer's Sparrow were 15 (n = 9) and 12 (n = 7) days, 14 (n = 5) and 10 (n = 7) days, and 11 (n = 1) and 9 (n = 1) days respectively. Sage Thrashers (n = 49) and Sage Sparrows (n = 17) had a similar probability of nesting success (0.45 and 0.40, respectively), while the Mayfield success rate for Brewer's Sparrows (n = 7) was only 0.09. Male Sage Sparrows that attracted mates had established larger territories than those that failed to mate. Brewer's Sparrows nested about 10 days later than the other species, which may have resulted in their lower nesting success, since nest site requirements of all species were similar. The Sage Thrasher, Sage Sparrow, and Brewer's Sparrow are common passerine species nesting in the sagebrush-dominated habitat on the Snake River Plain in southeastern Idaho. Each of these is said to be almost completely dependent on the sagebrush environment for survival (Braun et al. 1976). Until recently (Trost et al. 1975, Schroeder and Sturges 1975, Reynolds 1978, Rich 1977, 1978, Reynolds and Rich 1978), the ecology of passerines restricted to this habitat had received limited attention (Bent 1948, Feist 1968, Miller 1968, Paine 1968, Walcheck 1970, and Best 1972). Nesting data have been reported for the thrasher but they are lacking or incomplete for the two sparrows. I present here my findings on the territory size, nest placement, incubation and nesting periods, and the nesting success of these three species. MATERIALS AND METHODS Most of the data were collected from mid-March to mid-July 1976 and 1977, in two 4-ha study areas on the Idaho National Engineering Laboratory (INEL) Site, approximately 48 km west of Idaho Falls, Bonneville Co., Idaho. These areas were about 12 km apart and located in sagebrush steppe habitat (Kiichler 1964) dominated by big sagebrush (Artemisia tridentata; Harniss and West 1973). Data on nest placement and development of nests found elsewhere on the INEL Site were included, but territories were measured only on the two study areas. Reynolds and Rich (1978) presented preliminary data on the nesting ecology of the Sage Thrasher, which were gathered in 1976 on one of these study sites, plus an additional study area 25 km east of the INEL Site. Where appropriate, and unless stated otherwise, data from the present study were combined and presented with theirs in order to increase the sample size for this species. Territories were mapped for displaying males of each species following the methods of Williamson (1964) and Wiens (1973), and territory sizes were calculated according to Reynolds and Rich (1978). I considered males to be territorial only after their territories had been mapped at least three times on successive days. Although territories change in size and shape during the breeding season, the data presented here represent the total area in which territorial displays were recorded for each singing male, and thus represent maximum territory size. Wooden stakes, 1.8-m tall and color-coded with plastic flagging, were placed at 50-m intervals in each study area to ensure accurate mapping of territories and nest sites. Field investigators found nests by observing courtship, nest building, or food carrying behavior, or by flushing a bird from the nest. Birds were flushed by randomly searching the area or by two researchers dragging a 25-m rope with flagging material suspended from it across the top of the vegetation. Nests were examined at 1-3 day intervals. The condition of the nest and the number of eggs and/or nestlings present at each visit were recorded. Hatching for these species was, for the most part, synchronous. Incubation periods were recorded as the time in days elapsed between the date the last egg of a clutch was laid and hatching. Fledging also was generally synchronous, and nesting periods were measured as the number of days between hatching and fledging, recorded for each nest. Nesting success was calculated using the nest-day, egg-day, and nestling-day as units of exposure (Mayfield 1975),