An observer-specific, full-season, strip-map method for censusing songbird communities

-In this paper I describe and demonstrate a strip-map census method designed to provide a complete record of the composition and density of a temperate-zone breeding bird community through a period covering the peaks of breeding activity of all resident species and individuals. A route transecting the area of interest is laid out, flanked on each side by an unmarked strip of a width equal to the distance at which birds can be detected (the detection-threshold distance). All birds detected during uniform listening periods on frequent, closely scheduled, early morning traverses of the route are recorded with appropriate symbols at the moment of detection on simple line maps of the route. Because species differ in the distance at which their calls and songs can be heard and observers differ in their hearing acuity, each observer has an observer-specific strip width for each species. These strip widths are determined directly from the distribution of all detection points on the census maps at the end of the season. Multiplied by the length of the strip, they give the area of the strip for each species. The accumulations of detection-point clusters on summary maps for each species are interpreted as the territories of individual birds. Densities within the study area or segments of it are determined by dividing the number of equivalent territories (Q wholes plus fractions) within the species-detection strip by the area of the strip. Song frequencies are determined for each individual territory holder by multiplying the proportion of listening periods during which the bird was heard by the proportion of that bird's territory that fell within the species' detectability strip. The method provides a continuous record, divisible into segments of any desired length, of the density, song activity, and distribution of individuals of each species through its breeding season. The detection-threshold distance and song-frequency values obtained can be used as species-specific conversion factors for translating simple detection counts to bird densities. Received 3 October 1983, accepted 9 March 1984. FACING an increasing demand for quantitative data on songbird populations for scientific and applied ecological studies (Ralph and Scott 1981, Dawson and Verner in press), ornithologists have depended heavily on two relatively inefficient and unreliable census methods: short-term spot mapping (Kendeigh 1944, Anon. 1969) and converted detection counts (Emlen 1971, Shields 1979, Reynolds et al. 1980). With spot-mapping, as standardized for general application (Anon. 1969), one attempts to plot the home ranges of all singing males on small but representative plots during a brief segment of the breeding season when most resident birds are stably localized and vocally conspicuous. With detection counts, one attempts to record all visible or audible birds along a transect route or at a series of observation stations; such counts can, if desired, be converted to density values (birds per unit of area) by applying areal denominators and indices of cueproduction frequency. The usefulness of spot mapping is limited primarily by the small area that can be covered by an observer in a reasonable period of time (Svensson 1978), the variation among species and individuals in the period of breeding activity within which censuses should be concentrated (Best 1981), and the subjectivity of procedures used in interpreting clusters of song-detection points as individual breeding territories (Svensson 1974, Best 1975, Oelke 1981). The usefulness of the generally more efficient detection-count method is limited primarily by problems of obtaining reliable values for areas of detectability (areas covered by the observers' hearing and vision) (Dejong and Emlen in prep.) and for rates of cue production (songs, calls, etc.) (Emlen 1977, Christman in press). 730 The Auk 101: 730-740. October 1984 This content downloaded from 157.55.39.157 on Fri, 08 Jul 2016 05:33:55 UTC All use subject to http://about.jstor.org/terms October 1984] Strip-map Census Method 731 In this paper I describe a strip-map method that resembles the conventional spot-mapping method but differs in: (a) replacing the usual short-duration census period (designed to minimize the confusion of constantly changing individual activity and distribution patterns) with a continuous, season-long operation, and (b) replacing the usual, arbitrarily bounded, quadrangular plot with an elongate strip or series of strips of widths equal to twice the observer's detectability range for each bird species. For direct censusing, this method is obviously more time-consuming than spot-mapping; its advantages lie in the opportunity it provides for tracing changes in densities, composition, and individual distributions through the season and the greater completeness that results from frequent, regularly scheduled traverses of the area. It fills a need recognized by Best (1981) for combining intensive coverage with a temporal spread that includes the widely differing optimal periods of song activity of all species and individuals resident in the census area. In addition, it generates species-specific values for detection-threshold distances (and hence areas of detectability) and for cue-production rates, the two functions required for converting transect or point counts to population densities.