Not that it should matter all that much whether cormorants have increased or, as the evidence indicates, decreased since earlier times. Virtually no species of North American bird exists at the same number as it did at some point when there was, presumably, the “right” number. Apart from those who are now extinct, all exist at greater or lesser numbers than at some pre-colonial point in time, their numbers reflecting the ever-changing ability of the environment to support them.
But wildlife managers often seem to believe that if they can convince people that a common species is much more common now than it was in pre-colonial America, then there is something “wrong,” that the primal numbers are in some way the “right” number, and that an increase above that number indicates an error that must be adjusted. A decline in natural predators often is cited as the reason for the increase in whatever species they are targeting.
That argument does not apply to cormorants. There is no indication that that any natural predator ever had a significant impact on cormorant numbers.4 What does influence cormorant population size is the number of suitable nest sites available, and they are, if anything, much fewer than in pre-colonial North America. In Toronto, where CDI got its start, cormorants now nest on a man-made spit of clean landfill that extends miles out from the shore into Lake Ontario. While adult cormorants are relatively secure from predation, nests with eggs and young are vulnerable, and studies using “camera traps” have shown that the main predator of tree-nesting cormorants are raccoons. Raccoons are most abundant in urban environments, so to the degree (very small within the context of the cormorant population overall, since birds tend to nest on racoon-free islands) that predators are a controlling factor, they have increased, not decreased, overall in urban and suburban regions.
Ground nesting cormorants do not seem to be vulnerable to raccoons, at least at these colonies, and the possibility is that the ground nesting ring-billed gulls who coexist with the cormorants provide so much food for the raccoons that they never reach the cormorant nests. Coyotes, too, might prey upon cormorants, and be strong enough to take maturing young and adults, but their numbers, too, have increased, and their range has expanded eastward. Thus, the “loss of predator” argument simply does not apply to cormorants.