You asked: Why are we doing this? Will it hurt the eagles? Will they still be able to reproduce? How much does the transmitter weigh? Read on for our answers.
Why are you doing this?
Our eagles are part of a larger longitudinal study to investigate and learn about the lives of bald eagles. Our eagles are extra special because we know their place of origin. Most eagles that Brett traps are wintering along the Mississippi river. We think they are from Canada, but we don’t know that for sure: many of them nest in Canada in the summer. Since we know where our eagles come from, their behavior helps illuminate the life histories of bald eagles in general. What have we learned so far? We know that our eagles have a pronounced affinity for rivers (even small ones!), spend time in the company of other eagles, show fidelity to winter and summer grounds, improve the efficiency of migratory routes, and often summer in Canada. Eagles are a key umbrella species. Knowing what they require to survive and thrive through their wandering years helps us to preserve habitat and resources that benefit eagles and a lot of other species. If you would like to learn more about the study, follow this link: https://www.raptorresource.org/2019/04/08/satellite-tracking-decorah-iowa-fledgling-bald-eagles-2011-2018/.
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 eagle biologists. There are a lot of 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 backpack? 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 our eagles aren’t bothered at all by the backpack? We can’t. But after a brief period of nibbling around and exploring it, they ignore it – although they will preen the antenna like any other feather. Their parents don’t reject them, their siblings don’t reject them (although they might allopreen the new ‘feather’), and other eagles don’t reject them. The backpack doesn’t impede flight, feeding, socialization, or travel.
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, 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: in fact, it is well underneath the maximum. Let’s do the math! For a male bald eagle:
- Take 8.5 pounds and multiply by 16 to convert to ounces: 8.5lbs (average weight) x 16 = 136 ounces
- Take 136 and multiply by 28.3 to convert to grams: 136 x 28.3 = 3848
- Divide 55 (the weight of the backpack/transmitter in grams) by 3848 (the eagles estimated body weight in grams) for the decimal ratio of transmitter weight to body weight. Multiply by 100: 55/3848 = .01429 | .01429 X 100 = 1.429
The transmitter is about 1.429 of an average male bald eagle’s weight. Use 11.5 pounds for a for a female.
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. Many people have expressed concern that that the antenna might function like a lightning or grounding rod, putting the eagle at a higher risk for electrocution. But the transmitter does not provide a path to ground for lightning or change the eagle’s electrical potential. Lightning or grounding rods are taller than the things they are sitting on and provide a very firm connection to ground. Like water, electricity has to have somewhere to come from and somewhere to flow to. The trees that eagles roost, perch, and nest in are already very firmly connected to ground through their root systems: adding a 7.75 inch long antenna does not change the eagle or the tree’s connection to ground, or the path by which electricity might travel to get there. When an eagle is flying, it isn’t grounded, so the antenna (which isn’t connected to ground in and of itself) provides no path to ground at all. Curious about the antenna itself? Click here: https://raptorresource.blogspot.com/2016/08/eagle-tracking-can-you-do-something.html.
Why two antennas?
One transmits information to the satellite network that allows us to track eagles throughout the world. The other is a VHF antenna that allows us to track eagles locally. The first lets us see where they go. The second allows us to find them, take photos, and assess general health.
But some of these eagles have been electrocuted! This is YOUR FAULT!
If only! Than all we would have to do to stop the electrocution of bald eagles is quit placing transmitters on them. Problem solved!
Unfortunately, electrocution is a common source of mortality for bald and golden eagles. A massive study done by the Wildlife Health Center on bald eagle carcasses submitted to the National Wildlife Health Center in Madison, USA, between 1982 and 2013, found that electrocution was the third biggest source of mortality for bald eagles (poison was number one and trauma was number two) and the second biggest source of mortality for golden eagles.
Our transmitters have not caused the electrocution of eagles, but they have have saved lives by making clear that some lines and poles are dangerous. Power companies have responded by fixing them to prevent further electrocutions. Not knowing about or understanding a problem might fix your concerns, but it doesn’t fix the problem. We can’t fix what we don’t know about.
How do you catch the eagles?
I’m not going to get into the details, but we capture the eagles with something very similar to a Pa-dam trap, which is commonly used to catch birds of prey. It does not have moving parts or pinch the eagle (ie, it is called a trap, but it is not the leg trap that most people picture and does not work like a leg trap). Once the eagle is captured, the team immediately recovers it – members of the team do not leave the area once the trap is prepared – and secure it with a hood. This keeps the eagle and members of the team safe. As you might imagine, a lot of experience and training are required to obtain permits to do this. Brett and Carol trained under the Raptor Center at the University of Minnesota and have greatly refined their techniques (and trained others) since that time.
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 custom-fitted by the transmitter team.
How long will the transmitter last?
We don’t know for sure. Our transmitters are solar powered. Northstar’s website lists the operational life as indefinite, but probably 2-4 years: Brett has 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. The final stitching of the harness allows us to custom-fit it to the individual eagle’s body. We do not cut or pierce the eagle’s body, or insert anything under its skin, and we do our very best to reduce stress and ensure its safety during handling.
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 Brett to determine the best and safest type. He and his team have 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. Our study of bald eagles is not harming the eaglets in the short run and will significantly benefit Bald eagles and other animals in the long run.
Thank you for watching, sharing, learning, and especially for caring. It’s not easy to love a bald eagle or any wild animal, but our lives are immeasurably richer because people cared and acted: https://raptorresource.blogspot.com/2016/08/celebrating-centennial-100-years-of.html.
The Raptor Resource Project