Differential Response of Migratory Guilds of Birds to Park Area and Urbanization

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Urban Ecosystems

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Variation in species richness and density of native birds in urban parks and greenspaces (“parks”) is often substantial. Understanding why differences exist, and whether all migratory guilds are equally affected, is poorly known. We surveyed breeding bird communities in 48 undeveloped forested parks in Portland, Oregon, USA, to determine the contributions of park area, shape, connectivity, landscape composition surrounding parks, and differences in structure/composition of local habitat to variation in richness and density of residents, long-distance migrants, and short-distance/partial migrants. Migratory guilds responded differently to environmental factors. Richness and density of long-distance migrants increased with park area and abundance of small (< 10 cm DBH), mostly native, tree species. Resident richness also increased with the abundance of small trees. However, resident and short-distance/partial migrant richness was independent of park area, and resident density declined with increasing area. Park shape, connectivity, and landscape composition did not influence richness or density of any migratory guilds, possibly because of relatively high tree cover in Portland’s landscape. Separate analyses of forest-dependent species of all migratory guilds revealed that area was the primary contributor to variation in density of residents and long-distance migrants, structural habitat features contributed to variation in density of residents but not long-distance migrants, and that density of long-distance migrants declined with elongated park shape. Few forest-dependent species existed in parks below 10 ha, and their minimum area requirements for maintaining populations were estimated to be 30 to 40 ha. Without such parks most long-distance migrants would likely disappear from Portland’s landscape.


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