Bald eagles are nesting in more than 100 locations across Pennsylvania for the first time in more than a century, the state Game Commission announced in July.Todd Katzner, National Aviary
Eagle chicks photographed during field research on Eastern Imperial Eagle population in the Naurzum National Nature Reserve in north-central Kazakhstan.
Click photo for larger image.
Although there's nothing to suggest that the resurgent national bird's comeback in the Keystone State is anything but a dramatic success, a new scientific report has found that eagles and other endangered or threatened raptor populations could crash without warning because traditional measures -- like counting nests or mating pairs -- don't provide a complete picture of a species' health.
Looking only at nesting birds ignores "floaters" or single, non-reproducing birds, and adult mortality, and could lead to false and potentially fatal conclusions about the stability of critically endangered species, according to Todd Katzner, director of conservation and field research at the National Aviary on the North Side and co-author of the report.
"Just monitoring nests is not enough to identify potential threats that a long-lived raptor population may be facing," said Mr. Katzner. "This study suggests that we might want to take a more comprehensive approach."
The research paper, published this month in the scientific journal Conservation Biology, is based on research from a 25-year study of raptors in the Naurzum National Nature Reserve in north-central Kazakhstan.
Funded by the U.S. National Science Foundation and the Wildlife Conservation Society, Mr. Katzner and fellow study authors E.J. Milner-Gulland and Evgeny Bragin monitored Eastern imperial eagles, a species on national and international "Red Lists" of endangered birds threatened by habitat loss, scarcity of natural food sources and possibly environmental poisons.
The study used mathematical modeling for the first time to simulate the growth and decline of eagle populations, and was supported by use of noninvasive bird identification techniques, including the non-invasive collection and genetic evaluation of feathers in nesting areas to determine if birds were old or new nesting pairs.
Mr. Katzner said nest counts showed a stable population, but genetic tests revealed that adults were dying young and were being replaced by new birds from the "floater" populations. Such unattached, non-mating birds can comprise up to half of the total eagle population.
"So if you're just monitoring nests or territories you won't know what's going on in non-breeding populations and you could lose up to 40 percent of the population and never know it," Mr. Katzner said. "And if you lose that buffer you will have quick population decline."
He said that in Kazakhstan it would be appropriate to monitor the nests and adult survivor rates.
"If adult survivorship is low, something is going on that we should investigate," Mr. Katzner said. "Monitoring territory gives the best perspective on long-term threats but not short-term problems like pesticide poisoning, shootings."
Another example is the Swainson's hawk, a large grasslands raptor native to Canada and the United States that seemed to have declining populations despite good breeding success. A lucky tip led biologists to discover that tens of thousands of the birds were dying in Argentina, where they winter each year, after eating grasshoppers killed by a highly potent insecticide.
"The situation with the Swainson's hawk shows that threats that impact birds in the short term are not captured by territory occupancy and productivity measurements in a timely way," Mr. Katzner said. "Overall the population may be changing dramatically and we might not know until it's too late to save the population."
The study's findings could affect the way conservationists, biologists and federal and state agencies track the populations of long-lived bird populations of concern.
The U.S. Fish and Wildlife Service announced in February that it would use breeding territories to determine if the bald eagle should be "delisted" or removed from the federal threatened species list because the number of breeding pairs in the U.S. has increased from 417 pairs in 1963 to more than 7,000 pairs this year. Loss of habitat, traditional food supplies and in the 1950s the use of the eggshell-thinning insecticide DDT caused the population to decline from the estimated 100,000 breeding pairs that existed when the first European settlers arrived in North America.
Dan Brauning, the state Game Commission's wildlife diversity section supervisor, said Mr. Katzner's findings are important, but he isn't inclined to change the way the state monitors its bald eagle populations. Not yet anyway.
Mr. Brauning said Pennsylvania continues to have a rapidly expanding bald eagle population -- an average 15 percent annual growth curve for the last 20 years. In that situation, young birds are a positive development and leave little doubt that the population is expanding, as it has since 1983 when there were only three known nesting pairs in the state.
That was the year the commission started a bald eagle reintroduction program, bringing in 12 eagles from wilderness nests in Saskatchewan, Canada. Over the next six years 76 more Canadian bald eagles were introduced in the state.
A year ago, Pennsylvania changed the bald eagle's state listing from endangered to threatened, the same as the bird's federal listing.
"The study findings are relevant to the Fish and Wildlife Service when it decides to delist the bald eagle because it needs to look at things on a higher level than we do right now," Mr. Brauning said. "But it won't be long before we're in that place.
"The game commission is standing tall on the great recovery of the bald eagle in the state but we don't want to let our guard down. We want to make sure they're sustainable, and to do that we might need to do the additional monitoring."
Todd Katzner, director of Conservation and Field Research at the National Aviary on the North Side, with two imperial eagle chicks in a nest at the Naurzum National Nature Reserve in Kazakhstan in 2000.
Click photo for larger image.
Don Hopey can be reached at firstname.lastname@example.org or 412-263-1983.