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Double-crested Cormorants Under Siege (spring 2008)

The Peaceful Parks Coalition began studying the impacts of egg oiling on Double-crested Cormorants in 2003. This was a parallel study to the Ontario government's ongoing oiling program in the North Channel and Georgian Bay to reduce the overall cormorant population by 8%-11%. In 2003 the Ontario government oiled 2940 nests. In 2004 it expanded the oiling program to 5000-7000 nests at a cost of $300,000 annually.

The government study continues today but its objectives have changed. It is testing harassment techniques and banding young cormorant chicks to map the birds' migration routes. The U.S. Fish and Wildlife Service is interested in this information. They are currently developing a culling program that would target cormorants throughout their migration route or 'flyway' leaving no place safe for these birds anywhere.

The first phase of the government study was focused on fisheries only. If cormorants are having a deleterious impact on certain fish species, then reducing the bird's numbers should show positive changes in fish abundance. They removed 100% of future cormorant generations from nesting colonies through egg oiling hoping to detect strong changes in fish species data. They were not monitoring the impacts of the oiling program on the adult cormorants.

We know cormorants are very sensitive to disturbance, and were concerned that oiling cormorant eggs might have unforeseen negative impacts on nesting adults. Therefore, we observed nesting cormorants throughout the summers of 2003 and 2004.

The final report for the 2003 summer's work can be viewed at Cormorant Study 2003. The report for 2004 for can be viewed at Coromorant Study 2004.

Alternative Techniques For Controlling Bird Populations

Harassment: Double-crested Cormorants are very sensitive birds that prefer to nest in quiet and remote islands or ‘spits’ near open water. Sometimes simple but continual harassment or disturbance such as yelling and clapping or the presence of humans is enough to permanently scare off the birds. While this may appear to be a harmless control method, it can cause such extreme stress in the wild birds that they abandon ideal nesting sites, not just for that particular nesting season, but permanently. It forces birds into less optimal habitat, and may contribute to nest failure. It may also cause loss of body mass due to stress, and compromise the bird’s overall vitality.

Egg Oiling: Egg oiling is a lethal method of population control because it aims to reduce the overall bird population. The objectives are equivalent to that of shooting but the results are achieved over a longer time period. The application of mineral oil on eggs suffocates the embryo inside, and fools the adult bird into believing their eggs are still alive. This keeps the adult sitting on its clutch and discourages re-nesting. (Cormorants will attempt to produce a second clutch if their first clutch fails. Nesting behaviour continues until summer days begin to wane - a natural marker for nesting birds that signals the end of the nesting season). Egg oiling controls bird populations by suppressing the production of hatchlings compounded by natural mortality of adults.

Double-crested Cormorants are dedicated parents and will not leave their nests during incubation. The effects of egg oiling on nesting adults can result in incubation behaviour being extended up to two extra weeks. Average incubation period is 3.5 weeks. While sitting on their nests, adults do not feed, do not drink, and are susceptible to exposure. It interferes with natural reproductive instincts, and causes disturbances similar to that of harassment.

Nest Removal: This technique involves knocking nests out of trees with long poles or destroying ground nests. To avoid re-nesting attempts by cormorants, nest removal should occur late into the incubation period destroying both nests and eggs or chicks.

egg oilingKokanongi Shingle in July 2002 before oiling

egg oilingKokanongi Shingle in July 2003
after oiling

egg oiling spray paintOiled nest, marked with orange spray paint

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