Eagle Elder

We wrapped up the winter eagle trapping season last week with a pretty special capture. We had a good bit of success despite the fact that winter itself was extremely short, with good trapping weather (cold and/or snow) having ended by the first week of January. But whether there is weather or not Golden Eagles still have to eat, and carcasses are the mainstay in Central Oregon until marmots and ground squirrels come out of hibernation and birds and mammals start producing young.

At any rate, we caught a resident adult male, and found that he was already banded. So after taking some measurements, some pictures, and some blood (for a lead level study), we turned him loose.

As I filled out the Bird Banding Lab's band recovery form, I kept getting an error message; the band number I was entering was for a band that was deployed so long ago that it was unlikely to still be on a live bird. I don't have the full banding information yet, but the short version is that this male was banded as a nestling somewhere in our general area on June 6, 1996, almost 20 years ago!

Actually, the minimum age of this eagle (at time of our capture) was 19 years and 10 months. A check of the Bird Banding Lab's longevity records for this species shows our eagle to be the seventh oldest documented in the wild. He has a way to go yet to set the record, as a Utah Golden Eagle banded and subsequently encountered dead was a minimum of 31 years and 8 months old at death. Five of the six oldest birds were found dead, and the sixth--though released alive--was encountered only because it had run afoul of telephone or electric wires. So the eagle pictured below (with My son Nathan, his wife Nuka, and my granddaughter Celestine) is the oldest banded Golden Eagle to have been recaptured alive. Long may he yet live!

Trapping Golden Eagles

We're about at the end of another winter season of trapping Golden Eagles. It was a family affair again, and the photo above is of my daughter Willow holding an adult female. The season was short, as the weather has been unseasonably warm ever since the first week of January. (Our trapping success is greatest when the birds are stressed by cold and snow.) Fortunately, we had good success in December; we deployed PTTs (platform terminal transponders, i.e., solar-recharging, satellite transmitters) on four resident adults, and caught a couple more for banding and blood lead analysis. My son Nate is producing a short film about this work--go here to learn about or to contribute to that project.

A Near Score of Eagles

I had a couple of good days last week, days spent in a helicopter searching for nests of Golden Eagles (Aquila chrysaetos) in eastern Oregon (and a sliver of western Idaho). Such a search involves examining every rock and tree that could harbor the large stick nests that these eagles build; mostly, it means flying past and looking closely at a whole lot of rimrock and cliffs. The flights were timed to find females incubating eggs, in which situation they are very unlikely to fly, preferring instead to remain hunkered down and unmoving. (I'll fly again in late May or early June to determine the outcome at the nests found active last week.)

Along with the pilot--Paul McIlvain--I saw a good variety of wildlife. This included Mule Deer, Pronghorn, Bighorn Sheep, and Elk. The spike bulls and some of the 4- and 5-point Elk still carried their antlers, but the largest bulls had just dropped theirs. We saw a Raccoon sleeping in an abandoned hawk nest in the top of a Cottonwood tree, and we saw a huge black bear (on the Idaho side), one of the brown ones that make up about 30% of the Idaho population but which are much rarer in Oregon. We also watched a surprised Bobcat frantically seeking cover among the boulders at the base of a rimrock. (Cats are notoriously difficult to see from the air, as they generally find sufficient shelter at the first sign of an approaching helicopter.)

As for eagles, we found 19 active nests in the area we surveyed, an excellent total for a rather moderate and unassuming area of land. Some were associated with the Snake River and the abundant variety of potential prey that inhabits the area around such a watercourse. Others were in drier country where the only obvious prey base is Chuckar and Hungarian Partridge, both of which are introduced (non-native, and thus not historically-available) game bird species. Most of the eagle nests were on rimrocks or other large cliffs, but some were on smaller exposed rock outcroppings; one was on the wall of an old quarry, and one was in a Ponderosa Pine.

It's rough work, but somebody has to do it. (See if you can find the nest and eagle in the photograph below.)

More about Marmots

Two weeks ago today, I saw the first Yellow-bellied Marmot of the year, which I shared in a blog post a few days later. I mentioned their habit of estivating (going undergrown and entering torpor during the heat of summer) and then continuing straight into hibernation for the winter.

Anyone near agricultural areas will realize, however, that not all marmots estivate, that some remain active into the Fall and don't go underground to stay until it's time to hibernate. This difference in behavior is a response to irrigation. It's not so much the heat that causes normal populations to estivate--it's the lack of moisture. And so where farmers have the ability to keep the water flowing throughout the summer months, their crops will be an attraction to "rockchucks," especially younger ones (which typically stay out longer than adults anyway). In Central Oregon, these opportunists can be especially hard on alfalfa, as are the colonies of smaller rodents, Merriam's Ground Squirrels (Spermophilus canus) and Belding's Ground Squirrels (S. beldingi).

But whereas these large rodents can be a significant nuisance to farmers, they likewise play a significant role in the ecology of the region. I have firsthand knowledge of this from years spent climbing into the cliff nests of local Golden Eagles (Aquila chrysaetos).

Throughout much of the intermountain West, Golden Eagles are first and foremost predators of jackrabbits. But here in Central Oregon, we don't have the healthy populations of Black-tailed Jackrabbits (Lepus californicus) that used to be here. As a result, biologists from the Oregon Department of Fish and Wildlife wanted to know just what our Golden Eagles were eating. I was hired by them a few years back to climb or rappel into the nests of eagles to collect prey remains and regurgitated indigestibles to ascertain just that.

The result of those nest searches was a long list of species of mammals, birds, reptiles, and fish eaten by nestling Golden Eagles. The eagles in this area bring a wide variety of prey to feed their young, including most any animal large enough to be captured. (The size of an eagle's foot and talons precludes their capturing small perching birds and smaller mammals like chipmunks and mice.) But though the diversity of prey taken was the most important finding of that research, a secondary conclusion is pertinent to our present blog topic. In lieu of jackrabbits, Yellow-bellied Marmots appear to be the single most important prey--in terms of biomass--of Golden Eagles in our area during the breeding season.