The importance of native trees for forest bird conservation in tropical farmland.
Trees in farmland provide valuable ecosystem services that enhance agricultural productivity and income, as well as supporting biodiversity such as birds. A better understanding of the benefits of farmland trees for birds, specifically the relative value of native and exotic trees, is essential in developing effective management options, particularly within tropical regions with intense cultivation pressure. In farmland in central Uganda, neither total bird species richness nor richness of forest visitors (non-forest dependent) were related to any measures of tree cover, whereas richness of forest-dependent bird species showed a positive relationship with the number of native tree species. The density of 10 out of 17 forest-dependent bird species within farmland showed at least one significant relationship with measures of tree cover; variables relating to native trees typically had more, and stronger, positive effects on bird density than exotic trees. The combined density of 17 forest-dependent bird species on farmland was positively related to both the total number of native trees and number of large native trees. Increasing the density of forest-dependent bird species by 1 bird ha-1 within a farmland site is predicted to require c. one to two large native trees, or c. 30 native trees of all sizes, per ha. Despite comprising c. 40% of all trees, exotics exerted little positive influence on forest birds, possibly because these offer poorer resources in terms of foraging, nesting and shelter. While numerous previous studies in the tropics have considered the value, for birds, of tree cover in farmland, few have focused on tree size or species composition. This study suggests that the retention or planting of native trees is an important tool for conserving forest birds within farmland, will provide additional ecosystem services, and should be encouraged through, for example, agricultural development schemes, extension advice and demonstration farms.