Relationship between avian range limits and plant transition zones in Maine

Aim To determine if vegetation complexity associated with transition zones may be a contributing factor affecting bird species distributions in Maine, USA, and in increased numbers of bird species at about 45 degrees north latitude in northeastern North America. Location Maine, USA; North America north of Mexico. Methods We delineated the ranges within Maine (86,156 km(2)) of 186 bird species and 240 woody plants using literature and expert review. Maps showing species richness and numbers of range limits, at 324 km(2) resolution, were developed for woody plants and groups of breeding birds: forest specialists, forest generalists, and those that used barren and urban habitats, early successional areas, and wetlands or open water. Two plant transition zones for Maine were identified previously, with the north-south transition zone mapped across eastern North America. Patterns in bird distribution maps were compared to woody plant maps and to transition zones. Results When the distributions of forest specialists were compared to the north-south vegetation transition zone in Maine, they were spatially coincident, but were not for other groups. Forest specialists had more species with range limits in the state (61%) than generalists (13%) or any other group. At a continental-scale, the vegetation transition zone within eastern North America agreed fairly well with the areas of highest bird richness. Main conclusions A bird transition zone occurs in Maine and across eastern North America, akin to and overlapping the vegetation transition zone. Seasonality is likely the primary source of the inverse gradient in bird richness in the eastern USA, as reported by others. However, vegetation structure and habitat selection at very broad spatial scales appear to contribute to the reversed gradient. North of the vegetation transition zone, forest structure is simpler and coniferous forests more dominant, and this may contribute to reduced bird species richness. However, the northern (> 49 degrees) typical gradient in bird species richness has been related to many hypotheses, and several are likely involved in the genesis of the gradient.