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Birds of Prey
Population Trends
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.
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