Censusing Wintering Populations of Swainson's Warblers: Surveys in the Blue Mountains of Jamaica

-census methods developed for breeding populations of Nearctic-Neotropic migrant passerines are largely ineffective for determining the distribution and abundance of Swainson’ s Warbler (Limnothlypis swainsonii) on its wintering grounds in the Caribbean basin. Using playback of tape-recorded call notes interspersed with advertising songs, I found the warbler to be widespread and relatively common in montane forests of the Blue Mountains of Jamaica. Detection rates with playback varied from 17.8 to 29.2 warblers/l0 h along five census transects. Census efficiency was increased by an estimated factor of five to 10 times with the use of tape playback. Received 23 March 1995, accepted 20 Aug. 1995. For conservationists, the apparent decline of some Nearctic-Neotropic migratory birds is a serious concern (Terborgh 1989, Robbins et al. 1989) that demands both rigorous management and quantitative monitoring of breeding and wintering populations. Effective census techniques have been developed for many songbird species (Passeriformes) during the breeding season (Ralph and Scott 1981, Robbins et al. 1986). However, songbirds rarely sing on their wintering grounds and during fall migration, are feathered in duller basic plumages, and hence, are more difficult to detect and identify than during the breeding season. Yet most studies of migrants in the Neotropics employ censusing techniques designed for breeding birds of higher latitudes, with little or no attempt to compensate for the decreased detectability of wintering populations. As this report will communicate, standard point count and transect censusing methods can be relatively ineffective in determining the true abundance of secretive migratory species that winter in dense tropical habitats. The breeding and wintering biologies of Swainson’ s Warbler (Limnothlypis swainsonii) are among the most poorly known of the migratory wood warblers (cf Meanley 1971, Eddleman 1978, Graves 1992, Brown and Dixon 1994, Thomas 1994). Wintering birds have been reported in the Caribbean basin from the Bahamas, Cuba and the Isle of Pines, Jamaica, Cayman Islands, and the Yucatan Peninsula, casually east to Puerto Rico and the Virgin Islands, and from southern Veracruz south to Honduras (Meanley 1971, AOU 1983, Pashley 1988, Winker et al. 1992). Most wintering records are anecdotal (e.g., Eaton 1953), and, at present, wintering sites where more than one or two individuals can be consisDept. of Vertebrate Zoology, National Museum of Natural History, Smithsonian Institution, Washington, DC. 20560.