1961 Decennial Census of the Kirtland's Warbler

IN 1951, one hundred years after the discovery of the Kirtland's Warbler, Dendroica kirtlandii, 32 people participated with me in a count of singing males of the species throughout its nesting range (Mayfield, 1953).This was perhaps the first attempt of any group to census an entire species of songbird.In 1961 we repeated the count in exactly the same way, with 48 participants.The number of male Kirtland's Warblers in 1961 was 502, as compared with 432 in 1951.This is a 16 per cent increase but leaves the total population of adult warblers still in the vicinity of 1,000 birds of both sexes --a perilously low number for a species of small migratory birds.METHOD This census was feasible because of the well-known habits of the bird.It nests on the ground among small pines (Christmas-tree size) growing in nearly homogeneous stands of considerable extent, usually more than 32 hectares (80 acres).The only pines producing the necessary conditions in natural growth are jack pines, Pinus banksiana; but where plantings of red pines, Pinus resinosa, duplicate the conditions, the birds have sometimes accepted these also.The loud and distinctive song of the Kirtland's Warbler carries on a quiet day as much as two-fifths km (one-fourth mile).In good weather on June mornings, a male on territory sings persistently; the probability of song from a male on territory in a five-minute period is 85 per cent, according to data on eight males I have timed in various stages of the nesting process (Mayfield, 1960: 134).Since individuals tend to cluster in chosen areas, isolated males rarely occur, and a "colony" is seldom without song.The terrain is mostly level or gently rolling, with fire lanes or roads on most section boundaries and with sand trails suitable for cars leading into many of the sections.And, most important of all, many competent observers in Michigan know the bird well and are willing to put great effort into a census of this kind.Because of this knowledge it was possible to limit the area of search to reasonable proportions.The investigation covered the pinelands in and adjacent to the 12 counties in northern Lower Michigan (Mayfield, 1960: 10) where Kirtland's Warblers have been found nesting since the discovery of the breeding ground in 1903.Here we focused our attention on lands where (1) Kirtland's Warblers have been seen within the last 20 years, (2) pines have been planted within the last 20 years, or (3) forest 173 The Auk, 70.' 173-182.