Based on our observations and those of others from the literature, we construct a graphical model of habitat use in territorial species at high densities relative to optimal habitat availability. This model ignores differences in abundance among habitats, and, together with other models of habitat use, predicts that there should be greater stability (lower turnover rates) among individuals occupying optimal habitat than among those in suboptimal habitat(s). Future studies assessing quality among habitats might take advantage of this by comparing individual turnover rates among habitat types using standard mark-recapture methodology. As an illustrative example, we present a case in wintering wood thrushes (Catharus mustelinus; Muscicapidae: Turdinae) in which relative abundance and habitat quality were inversely related. Many individuals of this nearctic-neotropic migrant species hold nonbreeding territories in the seemingly crowded rainforest of southern Veracruz, Mexico.

Abstract. The Wood Thrush Hylocichla mustelina (Muscicapidae: Turdinae) was studied on wintering grounds in the rainforest of southern Veracruz, Mexico, using mist netting, radiotelemetry, and field observation. Data were collected during three boreal winters (primarily November-March), and 46 individuals received transmitters. All sex and age classes were present, but difficulties in aging and sexing did not allow accurate determination of proportions. Telemetry showed two types of spatial use: "sedentary" (n = 18) and "wandering" (n = 21). Wanderers moved greater relative distances, had higher subcutaneous fat reserves, and seemed to incur greater mortality then sedentary birds. Analysis of eight specimens suggests that these two strategies are practiced by both sexes. Six birds that received transmitters remained unclassified due to insufficient data. One isolated bird showed intermediate behavior. Netting revealed a lack of sedentariness in part of the population throughout the winter. Movements were correlated with weather changes: cold, wet weather systems brought an influx of transients into the lowland forest site. Vocalization and playback data imply that during these weather systems aggression subsided. Home-range size of sedentary birds averaged 0.44 ha (0.12-1.03 ha). Some wanderers seemed to make movements of >2 km. Wanderers were estimated to constitute approximately 50% of the population. Because wanderers apparently occupy habitats which undergo intolerable fluctuations in suitabihty and appear to suffer higher mortality,we tentatively accept Brown's (1969) critical density level 3 as a valid descriptor of wintering Wood Thrush densities in this region.

No abstract

No abstract

How avifauna respond to the long-term loss and fragmentation of tropical forests is a critical issue in biodiversity management. We use data from over 30 years to gain insights into such changes in the northernmost Neotropical rainforest in the Sierra de Los Tuxtlas of southern Veracruz, Mexico. This region has been extensively deforested over the past half-century. The Estación de Biología Tropical Los Tuxtlas, of the Universidad Nacional Autónoma de México (UNAM), protects a 640 ha tract of lowland forest. It became relatively isolated from other forested tracts between 1975 and 1985, but it retains a corridor of forest to more extensive forests at higher elevations on Volcán San Martín. Most deforestation in this area occurred during the 1970s and early 1980s. Forest birds were sampled on the station and surrounding areas using mist nets during eight non-breeding seasons from 1973 to 2004 (though in some seasons netting extended into the local breeding season for some species). Our data suggested extirpations or declines in 12 species of birds subject to capture in mist nets. Six of the eight species no longer present were captured in 1992–95, but not in 2003–2004. Presence/absence data from netting and observational data suggested that another four low-density species also disappeared since sampling began. This indicates a substantial time lag between the loss of habitat and the apparent extirpation of these species. Delayed species loss and the heterogeneous nature of the species affected will be important factors in tropical forest management and conservation.