by Barbi Lazarus
Double-crested Cormorants (Phalacrocorax auritus) are large, black birds that are named for the two small tufts of feathers on either side of their head. They are beautiful birds, with bright emerald green irises, blue, yellow or orange eyelids, and blue interiors of their mouths. They’re often easy to recognize by their habit of orienting themselves to the sun and spreading their wings in order to dry their feathers, and in some cases, to help regulate heat gain or loss.
Although there are thirty species of cormorants world-wide, there are only six in North America. Of these six species, the double-crested cormorant is not only the most widespread, but also the only one commonly found around inland fresh bodies of water.
Double-created cormorants feed primarily on small, and largely non-native and non-commercial fish, helping to keep populations of unwanted fish species in check. The majority of their diet is the non-native, invasive Alewife and Round Gobies, and they also commonly feed on abundant species like rainbow smelt and yellow perch.
Cormorants build large, shallow nests in trees and on the ground. They will gradually kill the trees they nest in through the deposition of their guano and by breaking branches for use in their nests. However, despite the damage these birds cause, the trees can continue to support them for long periods of time.
A Conservation Success Story?
While there are historical accounts of double-crested cormorants in North America as early as 1604, they have experienced dramatic population changes in recent decades. The widespread use of DDT caused a major population decline in North America, due to eggshell thinning, (meaning that the cormorants’ eggs would break before they had hatched and kill the embryos in the process), making them uncommon for some time. Double-crested cormorants were completely devastated by these chemicals, disappearing as a nesting species entirely on Lakes Michigan and Superior, and having only ten nesting pairs remaining on Lake Ontario at one point.
However, a ban on the use of DDT, as well as changes in the availability of the species’ prey base, led to an enormous population growth. From 1973 to 1993, the double-crested cormorant population witnessed a 300-fold increase to 38,000 pairs!
While this recovery is celebrated by some as an ecological miracle and success, it has caused great fear among many others, and has led to increasing cormorant and human conflicts (see Cormorant FAQs above). Some levels of government and groups have called for mass slaughters of these birds based on the ecological concern that the excrement of double-crested cormorants damages the trees they nest in, including on sandspits in the Great Lakes region. However, as noted above, double-crested cormorants do not kill as many trees as is claimed, as they re-use the same nests year after year, until the trees become too weak to use anymore, and in fact they prefer to use ground sites for nesting when they are available and the colony is safe from disturbance.