Why are you doing this?
The eagles we attach transmitters to are part of a larger longitudinal study investigating and documenting the lives of Bald eagles. We are learning a great deal about Bald eagle travel, wintering, and even socialization. However, our eagles are extra special because we know their place of origin. The rest of the birds our eagle biologist traps are juveniles or adults wintering along the Mississippi river. We think they may be from Canada, but don't know that for sure: many of them nest in Canada in the summer. We know where our eagles come from and we know their life histories, so their behavior will help illuminate the life histories of Bald eagles in general. So far, we know that D1 has a pronounced affinity for rivers, seems to enjoy the company of other eagles, and makes long flights. She didn't winter much further south than her place of origin, returned to Decorah (if not the fish hatchery) a number of times, and traveled north in the summer. While eagles are often associated with large rivers and magnificent landscapes (stony crags, snow capped mountains, mile-wide river canyons), D1 spent a great deal of time on small river systems that may seem less significant to us, but were very important to her. The preservation of river systems, even small ones, are clearly important to the health and survivability of Bald eagles long term. We may think we know a great deal about Bald eagles, but D1 (and her future study mates) are teaching us how much remains to be learned. The transmitter is yielding data on a part of their lives - what Bob sometimes calls the wandering years - that could not be studied otherwise.
|10 Days in the life of D1. This data was collected between January and February of 2012 and shows D1 on the Little Turkey in NE Iowa. To make your own D1 map, go to http://www.raptorresource.org/maps/personal.php|
How do you know it doesn't bother the eagle? What about reproduction?
When we first started talking about putting a transmitter on one of the Decorah eagles, we did a lot of research and talked with an eagle biologist we knew. There are many ways transmitters can be attached to birds, and we wanted to make sure we used something with a good track record. Did it have a long history of use? Did the eagles studied have mortality rates significantly higher than what we know about Bald eagle mortality rates? Did eagles that died and were recovered show signs of damage or wear from the transmitter rig? Were we able to confirm that eagles wearing transmitters reproduced? In the end, we went with the backpack because it fit our criteria. Male and female eagles wearing it have been observed nesting and appear to live normal reproductive lives.
Can we say for sure that D1 or any other eagle isn't bothered at all by the transmitter? No, we can't. Shortly after D1 was fitted, she seemed to do a little extra preening around her shoulders, where the backpack straps were located. This soon ceased, however, indicating that any discomfort was short-lived. Her parents didn't reject her, her siblings didn't reject her, and other eagles haven't rejected her. She flew long distances and survived her first year, indicating that the backpack didn't impede flight, feeding, or travel.
What kind of trap will you use to catch the juvie?
We'll most likely use a pan dam or noose trap again: a metal hoop with monofilament nooses that snare the eagle's legs. This method has been used for centuries by Eastern falconers, since it is easy to build, easy to set, and quite safe.
We lure the eagle into the trap, which is roughly the size of a hula hoop, with a dead fish or two. As it comes into the trap, the snares entangle its legs. There are no snapping or moving parts to this trap: the trap mechanisms (snares) are passive, so they do not spring or slam shut. We don't want the eagle to fly away with the trap still attached, so we tether the trap with a strong springy cord, which serves as both a stopper and shock absorber if the eagle does try to fly away. We conceal ourselves nearby so we can get to the trap very quickly once it is caught.
Here is a video of last year's capture: http://www.iptv.org/iowaoutdoors/story.cfm/story/8612/iao_20110809_105_eagle/video
Who makes the transmitter? Was it custom-made for RRP?
The transmitter is made by North Star Science and Technology: http://www.northstarst.com/
It is a solar-powered PTT. We don't have a model number.
Some things are custom - for example, we directed how often the PTT should cycle on and off to conserve battery life while collecting the data we need, and used an appropriately-sized and weighted rig for a large bird like a bald eagle. Other things, like the design of the PTT itself, are standard in every case.
Here is a clip from last year of D1 with her transmitter on. We are using the same overall design this year.
Will you use short and long range transmitters?
We will. At this point, we are able to use one transmitter for both purposes, so the eagle will have one transmitter instead of two. Looking at the satellite data, it seems like it should be easy to find an eagle using long range only, but in our experience it is nearly impossible, especially when they are in heavy brush.
How much does the transmitter weigh?
The transmitter weighs 55 grams, which comes out to about 2 ounces. Iowa Bald eagles weigh roughly 8-12 pounds (we estimated 8-9 for Dad and 11-12 for Mom), so the transmitter weighs about 1.4% of an average male bald eagle's weight, and about 1.06% of an average female Bald eagle's weight. This is in accordance with the North American Banding Council's recommendation of 5% or less: indeed, it is well underneath the maximum.
Male Bald Eagle...
Take 8.5 pounds and multiply by 16 to convert to ounces.
- 8.5lbs (average weight) x 16 = 136 ounces
- 136 x 28.3 = 3848
- 55/3848 = .01429
- .01429 X 100 = 1.429
How long is the antennae?
The antenna is 7.75 inches. It protrudes from the rear of the backpack at a roughly 45 degree angle. It is thin, flexible, and made of metal. There was some worry last year that that the antenna might function like a lightning or grounding rod, putting the eagle at a higher risk for electrocution. This is not the case because...
- The transmitter does not provide a path to ground for lightning.
- The transmitter does not change the eagle's electrical potential.
How long will the transmitter last?
We don't know for sure. Our transmitter are solar powered. Northstar's website lists the operational life as indefinite, but probably 2-4 years: our eagle biologist had one last as long as seven years.
What's the transmitter back pack like?
It is somewhat like a child's backpack. The teflon straps or ribbons go in front of and behind both wings, and are stitched together in the front. The straps are fitted snugly underneath the eagle's contour feathers, which prevents chafing and feather damage. We don't try to trap the eagles until they have been on wing for a few weeks, which assures that they have gained flying prowess and their flight muscles are fully developed. The final stitching of the harness allows us to custom-fit it to the individual eagle's body.
This is not our set-up, but it will give you an idea of what the harness looks like: http://www.learner.org/jnorth/images/graphics/d-e/eagleW_xmitter.jpg
Do a Google image search for 'eagle transmitter harness' or 'eagle transmitter backpack' for more images.
When will it fall off?
The transmitter will not fall off. We did a great deal of research into transmitters and backpacks and worked closely with our eagle biologist to determine the best and safest type. He has had very good results with the system we are using now: eagles have carried them for a long time, traveled long distances, and reproduced with them. We decided we preferred a system that he knew worked and had data on over one we had no experience with.
The North American Banding Council has established a code of ethics for bird banders, and we comply by it: http://avescr.org/Descargas/NABC/English/Raptor_Manual.PDF. Our Board and Director are all passionate about birds of prey: Bob Anderson has been involved in falconry since the late 1960s and bird conservation since the early 1970s. We believe that our study of bald eagles is not harming the study subjects in the short run, and will significantly benefit Bald eagles and other animals in the long run.