Double-crested Cormorants Alter Forest Structure and Increase Damage Indices of Individual Trees on Island Habitats in Lake Erie

Abstract. 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 prefe...