Seasonal movements of small-island birds along habitat and elevation gradients highlights the conservation value of small-scale agroforests

Abstract

Knowledge on species distributions and seasonal movement patterns within landscapes is important for understanding their ecology and roles in agroecosystems, and is critical for enabling their effective conservation and monitoring, particularly in small-island states where conservation resources are limited. We undertook bird surveys to examine species distributions across a range of agricultural land-use intensities and elevations on the Caribbean island of Grenada by collecting data over 6 repeated monthly surveys (3 wet season, 3 dry season) at sites located in a variety of habitats spread over 6–300 m elevation. We observed a gradual upslope increase in bird species diversity and abundance from the mid-wet season to the late-dry season, as well as a movement from reduced-canopy habitats to closed-canopy habitats in the dry season. The mid-elevation hillsides of Grenada consist of a diverse mosaic of closed canopy secondary forest patches, interspersed among more open agroforests and small-scale cropping and grazed areas. These agroecosystems appear to provide important habitats to birds throughout the year, especially open canopy agroforests in the wet season. We also found that open mixed agroforests contained a greater diversity of fruit, flower, and seed resources than closed canopy habitats such as secondary forest, cacao agroforests, young secondary forests, or dry forest scrub; and that plant food diversity was highest in the wet season. However, bird abundance significantly increased in closed canopy forest habitats during the dry season, suggesting that intact canopy forests provide important dry season habitat for many Grenadian birds, which may be particularly important for species in drier lowland areas experiencing more intensive agricultural land use and urban and resort development. Overall, this research demonstrates the importance of maintaining the functional connectivity of treed habitats at local and landscape scales for species conservation, as well as the contribution of diverse, small-scale agriculture to maintaining small-island bird communities.