Carolinian habitats on Middle Island in the western basin of Lake Erie have recently experienced a dramatic rise in nesting Double-crested Cormorants (Phalacrocorax auritus). Nesting cormorants on the 0.3 × 1.1 km island increased from three pairs in 1987 to 4,690 pairs in 2006. The physical attributes of individual trees and poles were assessed using common indices of tree health to determine whether forest damage increased with cormorant nesting densities. Crown density, branch damage, foliage transparency and decay were measured at 54 sampling stations along twelve transects in June of 2004, 2005 and 2006. All damage indices except for tree crown density and pole decay class increased over time, with trees more damaged than poles. Nests were more than four times more likely to be found in superstory trees than overstory trees and were virtually absent from understory trees, suggesting that Double-crested Cormorants prefer larger trees as nesting sites. However, despite greater cormorant preference for large trees, understory and open canopy trees had significantly greater levels of foliage transparency compared to overstory trees. The spatial distribution of damage varied across the island. Stem damage was lower in the center of the island compared to the western and eastern sections. Branch damage and foliage transparency were also greater on the eastern edge of the island than in the center suggesting that impacts were not yet concentrated within the forest interior. Densities of Double-crested Cormorant nests per station were significantly related to all damage indices except decay class. Stations with high numbers of cormorant nests were more likely to have lower crown densities, more transparent foliage and greater branch damage than stations with fewer cormorant nests. The data suggested that the distributional variation in damage could be used to better target specific areas of the island for management such as deployment of sonic deterrent systems, egg oiling or culling.
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