A Review of Factors Limiting the Kirtland's Warbler on its Breeding Grounds

Abstract

Previous discussion about factors limiting the population of Kirtland's warbler has centered about nesting success and adult mortality in winter. I present a review and synthesis of published and unpublished reports on Kirtland's warbler demography which suggests that this species may also be regulated by habitat maturation and fragmentation, pairing success, fledgling mortality and breeding dispersal. Only 85 % of male Kirtland's warblers may pair successfully, though this low pairing success is offset somewhat by polygyny. Estimates of fledgling mortality (30%) and pairing success are combined with published data on Kirtland's warbler productivity (3.1 young/pair) and adult survivorship (75 %) to calculate annual recruitment of yearlings (28%) assuming a static population. Kirtland's warblers are concentrated into a few large breeding areas, each of which provides suitable habitat for only 10-14 years. The growth and decline of these colonies is described from a comprehensive, 13-year population count. For the short term, regenerating habitat may not be sufficient to replace currently occupied maturing stands, and a habitat shortfall is predicted for 1986-1987. INTRODUCTION The known nesting range of the Kirtland's warbler (Dendroica kirtlandii) is restricted to an area ca. 120 by 160 km in northern Lower Michigan. All nests have been found within 13 counties. But migrants and stray summer males have been collected and observed across a much broader range from Missouri to the SW, Minnesota to the NW and Virginia to the E (Tilghman, 1979). The quantity of suitable breeding habitat available to the Kirtland's warbler (KW) has decreased in recent decades (Ryel, 1981b). Typically, the species occupies dense 1.7 to 5.0-m-tall jack pine (Pinus banksiana) stands of wildfire origin. Plantations are also used, including a few red pine (P resinosa) stands, but logged, unburned jack pine stands stocked by natural regeneration from nonserotinous cones are usually not dense enough for breeding warblers. In 1951, Harold Mayfield organized the first census of the entire known population of the species (Mayfield, 1953). This census counted 432 males. The second census in 1961 discovered 502 male Kirtland's warblers (Mayfield, 1962). However, the third count taken in 1971 showed a 60% decrease to 201 males (Mayfield, 1972). The principal reason for this decline appeared to be nest parasitism by the brown-headed cowbird (Molothrus ater). Mayfield (1960) estimated the parasitism rate to be 55% and Walkinshaw and Faust (1974) estimated it to be 69%. They found that less than one Kirtland's warbler fledged per nest. In response to the dramatic population decline from 1961 to 1971, a Kirtland's warbler Recovery Team was appointed under authority of the Endangered Species Act of 1973. This team instituted the following steps to help the Kirtland's warbler: (a) annual population census: (b) cowbird control; (c) closure of breeding areas during the nesting season, and (d) an expanded habitat management program. I will address four major topics in this paper: (1) a synthesis of published and unpublished demographic data that postulates lower reproductive potential and higher rates of breeding dispersal and fledgling mortality for Kirtland's warblers than have been assumed previously; (2) the present concentration of Kirtland's warblers into only a few breeding areas, and the growth and decline of such colonies; (3) the relation between habitat quantity, decline of individual nesting areas and the total Kirtland's warbler population; (4) an overview