Population Trends

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The number of raptors in the NWT varies annually and this fluctuation is related primarily to prey abundance. Species that rely on a fluctuating prey base include rough-legged hawks and northern harriers (voles and lemmings), goshawks (snowshoe hares), and kestrels (insects and voles).

Raptors that eat mostly birds (peregrine falcons, sharp-shinned hawks, merlins) or fish (osprey, bald eagles) and species that hunt a broader variety of prey (Swainson's hawk, red-tailed hawk, gyrfalcon) have much more stable populations. An interesting discovery of the Rankin Inlet Peregrine Study was that tundra-dwelling peregrines eat a lot of small mammals and therefore fluctuate in density and reproduction more than other peregrine populations.

Between the 1960's and the early 1980's, raptors that relied upon migratory birds for most of their diet (peregrines, merlins, sharp-shinned hawks) experienced population declines all over North America, including the NWT. The culprit was DDT, a pesticide used throughout North, South and Central America. DDT persists in the body's fat cells and becomes more concentrated as it moves up the food chain. For example, if 10 sparrows each eat 10 grasshoppers contaminated with DDT, and a peregrine falcon eats the sparrows, the peregrine receives 100 grasshoppers worth of DDT.

This is one reason why top predators like raptors are so valuable; they are a good indicator of environmental problems. The DDT scare is now largely over, although Central and South America still use DDT to some degree. But that is no reason to become complacent. The recent die-off of Swainson's hawks in Argentina reminds us that there will always be new poisons and we must remain vigilant if we are to protect our environment.