Food supply seen as factor in eagle reproduction
Copyright 1998 Associated Press
September 8, 1998
Bald eagles living along Lake Superior raise fewer young than those nesting on
inland lakes because less food is available for them, researchers say.
A study by University of Wisconsin-Madison and state Department of Natural
Resources researchers found the problem to be environmental, rather than due to
feeding on contaminated fish as some had thought might be the case.
William Karasov, a UW-Madison wildlife ecologist, said he conducted the study
for two reasons:
- To determine how substances like the insecticide
DDT, which has been banned since 1972, and the chemical pollutant
PCBs, which have been the focus of environmental
cleanup efforts for years, could still affect eagle populations.
- To measure whether eagle populations can serve as a
"bio-sentinel," or indicator of overall environmental quality. He concluded that more study is
needed to determine what looking at numbers of eagles says about the
environment.
DDT is blamed for a steady drop in eagle numbers between 1940 and
1970 because the pesticide accumulates in the food chain and causes eggs to
have thinner shells.
No eagles nesting on Lake Superior produced young from 1970 to 1976, but since
then, numbers of eagles and other birds of prey have been on the rise. There
were 108 occupied eagle territories in Wisconsin
in 1993, compared with 645 last year, the report said.
In the case of eagles nesting along Lake Superior, Karasov's study found the
birds' blood contained DDE, a breakdown product of
DDT, and PCBs, but that wasn't believed to be causing them to raise fewer chicks.
Over the
course of the eight-year study, researchers found that eagles on the big lake
raised 23 percent fewer chicks than inland nesters.
But they found it was more a matter of how much food the birds caught than the
contaminants in their blood.
Lake Superior is
"a deep,
cold, rather unproductive lake with few shoals where fish can spawn and eagle
can forage successfully," Karasov said.
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