eWestern Birds

Estimating the Number of Territorial Males in Low-Density Populations of the Sooty Grouse

James D. Bland

ABSTRACT: Sierra Sooty Grouse (Dendragapus fuliginosus sierrae) are challenging to census because they occur at low densities, are cryptically colored, and live quietly in the forest canopy most of the year. I developed a census method that accounts for several aspects of Sierra Sooty Grouse breeding biology that hinder accurate estimates, including seasonality of singing, anomalous singing by yearling males, low population density, and clumped dispersion of breeding males. Within 167 km2 near Pinecrest, Tuolumne County, California, I conducted landscape-scale censuses along a network of line transects from 2006 to 2009 and detected 22 clusters of breeding males (hooting groups). I then used spot-mapping methods to estimate the number of individual males within hooting groups. Territorial display by transient (yearling) males lasted only a few days and became uncommon after 1 May; persistently territorial males became increasingly reluctant to display after mid-May. Thus limiting the census period to 1 May–15 June maximizes detections of persistently territorial males, and a minimum interval of 5 days between repeated censuses minimizes misidentification of transient males as territorial. In the 13 hooting groups that I spot-mapped, the number of persistently territorial males averaged 4.9, and the distance from the center of a territory to the center of the nearest neighboring territory averaged 209 m. The probability of a persistently territorial male being detected on a single census visit averaged 0.71. Three repetitions of the group-scale census within a hooting season were sufficient to detect 98% of persistently territorial males. The density of territorial males was much lower (~0.6 male/km2), and the distribution of males' territories was much more clumped, than reported in other regions. The number of persistently territorial males was static from 2009 to 2011.

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