Population Dynamics of the Wood Thrush in 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 suitability 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.