Monday, November 12, 2012

To Catch a Predator (Part 2)

Our snares are set, our net is at the ready, and we've been standing around in the snow since dawn, waiting for the eagles to come in. We keep a close eye on our snares and bait with binoculars and spotting scopes.

Each time an eagle nears a perch we've erected, or flies over the bait in front of our net launcher, we hold our breaths. Typically, the eagle moves on, or perches nearby. We sigh, and wait for the next close encounter.

Then, unexpectedly, an eagle flies down and lands next to the chum carcass set out in front of our ballistic net. It looks around, cautiously, before beginning to feed. We wait until the eagle has its head down, focused on tearing a nice, juicy bit of chum off the carcass, and then-- "Fire!" We trigger the ballistic net remotely, and the .22 gauge blanks power three padded projectiles, launching the net from its housing out and over the bird.

"We got it!"

Everything is a rush. We sprint from the roadside, where we've been watching, down the river embankment to our freshly-netted eagle. We must move quickly, to ensure that the eagle doesn't escape from the net, or get too tangled. Immediately, we secure the talons of the eagle in our hands, and place a hood over its eyes. Eagles receive most of their sensory information visually, so when they can see what we're doing, they get stressed. With the hood on, however, they immediately calm down, and are relatively docile in the hand. Unlike many mammals, birds do not need to be anesthetized during handling. The hood keeps the eagles calm enough.

We then begin the process of extracting the eagle from the net. Here, Steve Lewis and Dr. Scott Ford work the netting off the eagle's wings.

Steve and Scott work the netting off the eagle

Once the eagle is free, we add an additional element of protection: leather booties that cinch around the eagle's feet. An eagle's greatest weapon are its sharp, extremely powerful talons. Eagles can do a great deal of damage with their feet, and although the hood keeps them calm, they still grasp at anything that comes near their talons. They can still grip through the leather, but the booties help keep the talons from puncturing through the skin, should the eagle get hold of someone's hands or arms.

Scott puts leather boots around the bird's talons
After the bird is freed from the net and the boots are on, we give it a quick look-over to make sure the bird appears healthy and has no injuries.

Free from the net
Then we add one final measure of projection for the eagle: a canvass wrap that Velcros in place, securing the eagle's wings against its body. This helps ensure that the wings aren't damaged if the eagle shifts around during transport or the first few stages of the handling process.

Putting on the body wrap
Success! We caught an eagle! The eagle is free from the net and all the protective gear is in place. Now it's time to begin the processing stage.

This was the first bird we captured, a large female.
We take the eagle up to our vehicle to process, away from the trapping site and out of the snow. 

Ready for processing
First, we collect some measurements. Juvenile eagles are mottled brown, or have brown streaks in their head and tail feathers, while adults have solid white tails and heads. Female eagles are larger than males-- they weigh more and have larger talons and bills. We take a weight on our eagles and measure bill length, bill depth, and the length of the talon on the hind toe to determine the sex of the eagle.

Steve, taking a weight reading. The body wrap helps keep the eagle secure.

Yiwei, holding a juvenile eagle for measurements.

Measuring bill depth with digital calipers
After our measurements are complete, we put leg bands on the eagle. We use two leg bands, one on each leg, to help identify the bird. The US Geologic Survey (USGS) is in charge of all bird banding. They provide bands to all researchers who study wild birds. Every wild bird that is captured and handled should get a USGS leg band. These bands have unique identification numbers on them, as well as the phone number to the USGS Bird Banding Lab.

When a bird is banded, information about the species, sex, age, and capture location go into a database. Later on, if a bird is recaptured or is found dead somewhere, the bands can be read or recovered. The people that find the banded bird can call the USGS and learn where it was originally tagged, and can provide information about when and where the band was found. These USGS leg bands have provided valuable information about bird migrations, as well as habitat fidelity and species longevity. Leg bands can be difficult to recover, but those that are found help us learn about the ways birds move.

USGS leg band
In addition to the USGS band, we attached a colored, alphanumeric band to our eagles. These bright green bands have large numbers and letters, allowing them to be read from a spotting scope. This will help us resight our eagles in the Chilkat Valley, and will help us determine if a transmitter isn't working properly. If we read the band number on a bird from which we're no longer getting satellite data, we'll know that the transmitter has failed, but that the bird is still alive. Astute birders also sometimes sight and read these bands, and email us the information, letting us know that they've seen our birds and they're alive and well.

Attaching leg bands
The next step is the part of processing that takes the longest: fitting the satellite transmitters. We use solar-powered GPS units, mounted on a harness made of Teflon ribbon that the eagles wear like a backpack. These units collect GPS locations and information about altitude and temperature, and relay those data to a satellite system. The satellite system then feeds the information into a database online, where we can access and download the readings. The units take a GPS location once every hour from sunrise to sunset, and we receive the information from the satellites every two to four days. The units weigh very little. Eventually, the birds will preen their feathers around the harness and tracking device, so all that will be visible will be the antenna.

Tracking device and harness, during fitting
Once the harness is properly fitted (not too loose, not too tight, not rubbing anywhere, etc.) we secure it in place with rivets and clip off the excess ribbon.

Securing the harness
We glue the cut ends of the ribbon to prevent fraying, then clip the end off one wing feather to keep for chemical analyses.

Getting ready to clip a few centimeters off a wing feather
Once the harness is secure and we've collected all our measurements, the last steps are to take photographs to document each individual.

Documenting wingspan
Sometimes, with juvenile birds, we need to collect an extensive record of photographs to properly determine the bird's age. Bald eagles don't mature until five years of age, but until that point different molting patterns can be used to determine how old an individual is. Three of the birds we captured were adults. The other two were juveniles, so we took several photographs of the wings and tail feathers of those birds to help identify their ages.

Photo documenting molting pattern on a juvenile eagle's tail feathers
Processing, in total, takes around 45 minutes. We try to get the birds processed as quickly as possible to limit the stress of handling. Although the hoods help them remain calm, they still aren't too keen on being held and maneuvered. Finally, though, the leg bands are on, the tracking device is secure, and the eagle is ready to be released!


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At November 15, 2012 at 9:09 AM , Anonymous Anonymous said...

We're taking a look at your blog in our Rhetoric class and we think your work is AMAZING! You are truly an inspiration - thank you for sharing and keep it up!

At November 24, 2012 at 4:52 PM , Anonymous Anonymous said...

How long do the teflon straps last? I love the idea of being able to know where and how the eagle is doing, but bulk at the teflon straps. Do they all fall off or do some stay on the eagle even after the transmitter has quit working? Thanks for watching out for our eagle friends.

At November 24, 2012 at 10:26 PM , Blogger Rachel said...

Hi there,

The Teflon straps won't last indefinitely. Many eagles are able to "chew" through them and work them off anywhere from a few months to a few years after getting a transmitter. For those that don't, the Teflon will wear through. Depending on the eagle's preening habits and the environment (i.e. the amount of direct sunlight and moisture exposure), the straps will naturally wear and break apart after three to five years or so. The transmitters are supposed to last at least three years, but some can last upward of five or more. If a transmitter fails early, the harness will stay intact for a period of time, but typically the Teflon will fail before the transmitter itself does.


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