Bringing the raptors back--one man's passion

By Linda Bell
Correspondent

In 1972, when Jerry Craig began his long career as raptor biologist for the Colorado Division of Wildlife, there were no documented breeding pairs of bald eagles in the state. By the time he retired from the division in 2004, there were 50, including two in Larimer County. While formerly endangered peregrine falcons declined to only four breeding pairs in 1981, now there are 114 occupied nests in Colorado. What changed?

Public awareness, for one thing, Craig said, and a ban on DDT that caused shell thinning and poor reproductive success, for another. In the case of the peregrines, he said, the DOW created an active egg recovery and hatching scheme that put some wildlife biologist hanging by ropes eye to eye with a nesting female. Perched on the edge of high cliff, she was unaware the substitute eggs under her were made of plastic.

"The first time we exchanged those eggs for four live young--already three weeks old and half the female peregrine's size--we didn't know what would happen," Craig said. "The female challenged, then immediately settled down to brood the nestlings as if they'd been there all along."

The breeding program, Craig said, was based on hatching eggs from many captive peregrine pairs held at the division in addition to the naturally nesting birds. To increase numbers fairly rapidly, he said, biologists would take the first clutch and force the birds to lay a second one, providing lots of genetic diversity among the fertile eggs. The nestlings replaced in the wild were randomly selected from the entire group, he said.

Craig's work with peregrines led to many local, state and national awards and the creation of a DOW publication Craig wrote with James Enderson on "Peregrine Falcon Biology Management in Colorado 1973-2001."

Bald eagle recovery

"When I was a boy growing up in Colorado Springs," Craig said, "shooting eagles was expected behavior. When I joined the DOW in the early 1970s, eagles were still shot out of power lines. Active eradication of eagles was prevalent through the 1940s, 1950s and 1960s because they were thought to be preying on calves, lambs, Angora goat kids, anything. There was a lot of pressure to control eagles, either by shooting them or poisoning them."

Even though the eagle protection act that included both bald and golden eagles was passed by Congress in 1940 to conserve a bird declared the national emblem in 1782, law enforcement teeth and punitive fines weren't added to the act until 1972 when bald eagle populations started to plummet. They have been protected in the lower 48 states as either endangered or threatened under the Endangered Species Acts since 1967.

Eagles, along with osprey, brown pelicans and the peregrines, all suffered effects of shell thinning from DDT, Craig said. After DDT was banned and washed out of the ecosystem, the birds started to rebound. Eagles were reclassified as threatened instead of endangered in 1999.

In addition to Colorado's nesting resident population, Craig said, there is a sizable wintering population of more northerly nesting birds. They usually stay until about mid-March and fluctuate in number in Colorado depending on weather and food resources, he said. Each year since 1987, Colorado has conducted winter surveys of bald eagles in mid-January. Last year the count estimated 960 bald eagles wintered in Colorado, and the results for 2005 were similar, Craig said.

While all eagles are opportunistic in their diet, often preying on road kills and other carrion, bald eagles historically prey on waterfowl and fish, Craig said. Wintering birds roost socially in trees - sometimes as many as 100 in one small area, near natural ponds, around reservoirs and along major river drainages, he said.

"They've even learned over the years", Craig said, "that some of our reservoirs freeze and then get snow covered, shutting off oxygen to the fish. Some fish die, rise to top and turn into 'fish popsicles.' The eagles love them."

Bald eagles nest only in trees, he said. The DOW monitors all documented Colorado nests and bands the juveniles, including giving them numbered cattle ear tags that are large enough to identify from a distance. "This way we know their movements and how they are expanding throughout the state," he said.

Eagles mate for life, but that doesn't mean they don't pair again if they lose a mate, Craig said.

"In Colorado," Craig said, "we say our nesting bald eagles lay their eggs on Valentine's Day, the eggs hatch at Easter, and we band on Mother's Day."

Craig said at first, with so few nesting pairs in the state, they worried about viable genetic diversity, but recent DNA studies show that all bald eagles are genetically similar due to some cataclysmic episode that nearly crashed the population about 10,000 years ago.

Resident golden eagles

"Bald eagles are pussy cats compared to golden eagles," Craig said. "Goldens nest in high cliff faces or in trees and are definitely not as tolerant of people. That's why we've been seeing their populations decline over time along the Front Range and elsewhere."

Craig said there are two distinct populations of golden eagles in the state, one dependent on grassland small mammals like jack rabbits and prairie dogs. The other more resource-adaptive population lives in the conifer foothills and forages on magpies, porcupines, bull snakes, blue grouse, Wyoming ground squirrels and more. Both populations are losing habitat, he said.

Unfortunately for the foothills golden eagles, some people also like to build or hike close to the tops of cliffs or mountains, Craig said, which disturbs some of their traditional nesting sites, some over a thousand years old. The plains golden eagle populations are dependent on food resources that wink out periodically, either because of natural boom-and-bust cycles, or because of deliberate eradication for plague or pest control, he said.

Eagle viewing

Springtime is an ideal time to watch raptors, Craig said. Bald eagles are prominent in their northern migration, goldens hold their territory through mating displays of high altitude dives, and red tail pairs court in an aerial ballet, he said. Both eagle and red tail males help build up nests and can be seen hauling sticks, like proverbial boxes of chocolate, to impress their mates, he said.

"One of the most anthropomorphic moments I've ever had in the field was watching a bald eagle bring a huge stick to the nest, almost bonking his sitting mate on the head upon landing," Craig said. "When the stick was down, she retrieved it and ceremoniously dropped it out of the nest."