Effects of Habitat Use on Survival and Physiological Condition of Wintering Neotropical Migratory Birds in Belize
Some species of Neotropical migratory bird populations have experienced declines in recent years. Knowledge of the causes of population changes is critical to effective conservation efforts and migrant population declines are thought to be at least partially a result of habitat alteration on wintering grounds. However, effects of land-use in tropical forest ecosystems on migratory bird populations is poorly known. Migratory bird populations may be limited principally by factors affecting winter survival yet estimates of winter survival in different habitats are lacking. In this project, we examined the effects of tropical forest conversion on winter survival of migratory birds. We tested the statistical hypothesis that winter survivorship of understory migratory birds was not different between mature and early successional forest. We focused on 4 species of migrants common in both habitats in southern Belize: Wood Thrush ( Hylocichla mustelina), Ovenbird (Seiurus aurocapillus), Hooded Warbler ( Wilsonia citrina), and Kentucky Warbler (Oporornis formosus). We chose these species because 1) Wood Thrush and Kentucky Warbler have experienced recent population declines, while Ovenbird and Hooded Warbler have not, 2) Wood Thrush and Kentucky Warbler are considered forest specialists sensitive to forest alteration, while Ovenbird and Hooded Warbler are associated with both disturbed and undisturbed forest, and 3) Ovenbirds are more abundant in early successional forest, while the other 3 species are more abundant in mature forest. Our study area was within the coastal plain of the Maya mountains in Toledo District, southern Belize. Likelihood ratio tests for different monthly survival probabilities of birds inhabiting mature vs. successional forests provided no evidence of a difference for any of the 4 species. Likelihood ratio test results provided no evidence of differences in capture probability between the 2 habitats for 3 of the 4 species. There was evidence of lower capture probabilities in successional forest than in mature forest for Kentucky warblers. The monthly survival estimates for the 3 species were all around 0.90. Models incorporating time-specificity of survival were not selected using AIC, indicating the absence of evidence for differences between monthly survival during early vs. late winter. When these monthly survival estimates are extrapolated to correspond to a 12-month period, they yield annual rates ranging from 0.25 to 0.42. Annual survival estimates for neotropical migrants of this size are typically higher (0.42-0.67).
