Yesterday we went on a boat tour of the Toronto Harbourfront (organized by the Toronto Bay Redevelopment Initiative, or something like that). As part of the tour we came close to a portion of the Leslie Street Spit/Tommy Thompson Park which is completely infested with Cormorants. Apparently once these birds nest in a tree, they manage to kill the tree within six years.

This photo shows a few such trees which have no leaves remaining, but you can see the numbers of birds and nests in the silhouette.