Saturday, November 10, 2012

To Catch a Predator (Part 1)

We just finished six intense days of eagle captures! In the coming week, we'll be posting several entries and many photos of our adventures, so check back often. Aided greatly by raptor biologist Steve Lewis of the US Fish and Wildlife Service and Dr. Scott Ford, an avian veterinarian, our efforts netted five total eagles, so we were able to deploy all of our satellite tags!

Eagles surveying the Chilkat River

As the weather turns chillier and the rivers begin to freeze, more and more eagles are congregating along a small stretch of the Chilkat River that hosts open, flowing water. This two- to three-mile stretch of river stays unfrozen year-round as a result of unique geology-- groundwater percolating upward from natural springs near the confluence of the Tsirku, Klehini, and Chilkat Rivers maintains water temperatures just above freezing. Eagles come to this area to access chum and coho salmon. When rivers freeze elsewhere, this spot is one of the only places eagles can find a meal!

Chum in shallow water
You might imagine that, with so many eagles hanging out in one area, it wouldn't be too difficult to catch a few of them. Eagles are quite a bit wilier than they look. They're intelligent animals, suspicious of new things, and have excellent eyesight. Outsmarting them required us to use quite a few tricks!

Our trapping site
We used two methods to capture our eagles: perch snares and a ballistic net. We learned quickly that if the eagles saw us setting these traps up, they wouldn't come near them. We made sure to arrive at the trapping area an hour before sunrise every morning so that we could operate under the cover of darkness.

Perch snares are what we call passive traps, because they do not actively attract eagles to them. There is no bait to draw an eagle in-- we just have to rely on eagles coming by and deciding to take a rest. We attach wooden perches (made of red alder, in this case) to existing logs so that they look like a nice place to sit and survey the landscape, or digest a recent meal.

The perch snares have a spring-loaded loop draped over the perching area, and a flexible joint that bends under a bird's weight. In theory, when an eagle lands on the perch, the branch collapses, triggering the spring to activate and tighten the loop around the bird's foot or one of their talons. These snares are great because they don't frighten the eagles. In fact, there is really nothing about them that an eagle would associate with human activity (unless they get caught). When birds land, branches sometimes bounce under their weight or break off, so an eagle isn't scared off by the sensation of our perch snares collapsing. This is works in our favor because the snares don't capture an eagle every time. Eagles sometimes land on the perches without springing the snare, or don't land in the right spot to get caught by the loop. Nevertheless, we caught three of our five birds using this method.

Perch snare, loaded and ready for action
Our ballistic net uses .22 caliber blanks to launch a fine, lightweight net over a bird that is within capture range. To lure birds close to the ballistic nets, we used whole chum salmon carcasses from the river and sockeye salmon scraps. We placed these baits close enough to the trap so that when an eagle began feeding on them, the fired net would be able to trap the bird before it had time to fly away. Here is a video Dr. Ford recorded in Washington of a similar net firing, from a bird's point of view:

We had less success with the ballistic net because it's difficult to camouflage. Eagles would sometimes fly closely over our bait, examining the chum carcass, but would fly away without landing after seeing the casing that houses the net. We did capture two eagles using the ballistic net, but it required a lot of effort and a decent amount of luck. We're hoping to refine our camouflage techniques for our next trapping season-- the net could catch several birds at once if they weren't so suspicious of its presence.

The ballistic net, with bait set in front. We later camouflaged this better.
Although we captured five birds in six days, we spent the majority of our time trying to stay warm, waiting for eagles to show up, or cursing under our breath (or out loud) when the eagles escaped our traps. Nevertheless, we all feel privileged that we could spend time in an amazingly beautiful landscape watching these magnificent eagles (along with ravens, magpies, gulls, and waterfowl) engage in their daily lives.

A cold, snowy morning
In our next post, we'll describe how the processing stage works: what happens with the eagle once we have it in hand. Stay tuned!

Trapping location at dawn

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