Bald Eagle Research in New England

Hi, my name is Chris DeSorbo, I'm the Director of the Raptor Program at BRI. As many of you probably know, BRI has been studying mercury exposure in wildlife for several decades now. Recently, we've focused a substantial amount of time on evaluating mercury exposure and impacts on Maine's inland eagle population. After over three years of sampling and banding over 300 eaglets throughout the state, we've learned a lot about mercury threats to Maine's eagles - and have set the stage to learn a lot more about a range of topics. Now that birds are getting on nest again, it is a reminder that our time for eaglet banding and sampling is quickly approaching. Since folks are always tremendously interested in how and why we do this research, I thought I would try to outline some of that below. I will start today to give folks a sense of the “how,” and will follow up later with more info on the project background and findings.

A typical workweek starts by packing the truck the night before with a boat, motor, climbing equipment, blood sampling equipment, food, water, and lots of maps. Usually a late night or en route phone call with Charlie Todd (MDIFW) would update us on which birds were old enough for sampling (5-7 weeks of age) based on his aerial surveys the day or two before. Charlie is highly skilled at accurately aging birds from the air from a low-flying plane with a skilled pilot. Once we had our target nests lined up for that day, we (myself and typically 1-2 field assistants) would begin a long drive early in the morning heading for our first nest. We typically arrive at our first destination on a roadside, riverside, or boat launch 2-4 hours later. Many nests were not easily accessible from the road, however; many require hiking, boating, or a combination of the two to get to the base of the target nest tree. The canoe comes off the roof, motor mounted, spare gas can, a lot of heavy climbing and sampling gear is piled in the middle, and off we go. At this point we may be motoring along slowly for 5 minutes or 45 minutes. If we’re lucky, the nest is right along the water’s edge. If not, it’s deep in the interior of some wooded ridgeline, blowdown, or wetland. In the unlucky cases, we’re tromping through the woods with a map, compass, and GPS unit heading the general direction of the nest. We rarely have nest coordinates, as many are first-time visits. Once we’re in the general vicinity of the nest, we walk around the woods looking up the whole time. 95% of the nests are in big white pine trees, and many are not visible from the ground. Once we find the nest tree, I begin preparing my climbing gear and my assistant sets up a station to process birds on the ground that will be clear of falling branches. A picnic of banding and blood sampling supplies and datasheets are all spread out on a blanket on a clear, flat spot in the woods if we can find one. We use traditional arborist climbing gear and techniques to get up the trees. I’m on my way up the tree with rope dangling below me. Within 15 feet, I often leave all the mosquitoes and black flies behind to focus on the ground-based processing crew. Fifteen to 45 minutes later, I’m at the base of the nest. Many trees are 60-80 feet tall, with a few topping over 100 ft. Each tree is different, and I must say, many represent some of the most peaceful, scenic and tranquil places I’ve ever experienced (Can’t beat the company!).

Once the nest is reached, I set up shop to allow me to quickly lower the bird once captured to the processing crew that’s been waiting patiently on the ground among many biting insects. Slow movements and a sometimes lengthy period of allowing the birds to acclimate to my presence allows me to slowly grasp a nestling’s leg and then quickly tip them into my bag that’s splayed out in the nest. Within a minute or two, the bird is standing upright in a bag as it’s sent down through the branches to the ground.

Luckily for us, Bald Eagle adults are generally quite passive and do not attack climbers (unlike many other raptors). There is also little risk of the adults abandoning the nestlings due to our visit, if timed correctly. The Eagle Cam demonstrates best what these birds have gone through (since early March!) to get to the point that their young are 5-7 weeks of age, generally sometime in June. Think of the time, patience, endurance, and energy expended to incubate eggs, and care for young during blizzards, rainstorms, 90 degree days, and blackflies! Like many birds, the more time they’ve invested in their young, the less likely they are to abandon it due to intruders. This again demonstrates why nest visits too early in the season could be devastating without properly coordinated nest visits. Additionally, it is often the case that only one adult ever knows we were there! You gotta wonder what they think when they arrive “home” and see their young are now wearing “bracelets”…
As soon as the bag is within the grasp of the processing crew, the processing begins and we try to get the bird back into the nest as quickly as possible. The bag is unclipped from the rope and immediately weighed. The bird is slowly removed from the bag and placed on it’s feet for a few measurements and banding. Many eaglets are surprisingly calm at 5-7 weeks of age don’t necessarily have the power in their talons to do much harm if they happen to grab onto you. As Charlie often puts it, “they don’t yet know the weapons they’ve got.” We draw blood samples for contaminants and genetics analyses from a vein on the underside of the wing. A few breast feathers are clipped, a few more measurements, and the bird is back in the bag again ready to be sent back up the tree. A little advance notice from the ground crew often allows me enough time to send the second nestling (if there is one) down just as they are finishing the first. We do a quick trade; bird #2 arrives on the ground just as #1 is ready to go back up.

Within a couple minutes, the first bird is back in the nest no worse for the wear, but now displays it’s new leg bands. All birds in Maine for this project have been banded with one silver USFWS band, and one red band with a unique code that is often readable from a distance with a spotting scope and some luck. These red bands have allowed us to see where these nestlings go to after fledging and will continue to provide us with all kinds of data on Maine’s eagle population for decades to come. For example, birds banded as nestlings during a similar project conducted during 1991-1992 are still turning up throughout the state and elsewhere.

Once the second bird has been processed, it is returned to the nest with its sibling. Often times, I open the bag, and eaglets simply stumble out and take their place next to their sibling where I found them. The nestlings are always less tense to my presence after they’ve been put back in the nest, allowing for a quick photo before I duck back under the nest and start rappelling back down to the ground. When I’ve reached the ground, the processing crew is ready to go and we make our way back to the boat, then the car, and start thinking about the next nest up for the day. If it’s not a long drive, and we can potentially fit in 3 or rarely 4 nests in a day. The sun is usually setting by the time we are finishing our last nest for the day, and all we want then is hot food, a hot shower (you’re generally covered in sap and grime by the end of a day climbing trees), and sleep in a random motel hopefully close to where we plan to begin the following day. Unfortunately, there are samples to organize, label and process, and logistics to arrange for the following day. I have never slept so well as I do during this fieldwork.

This is the day in / day out routine of our eagle sampling crews for a roughly six week period during the summer. None of it would have been possible without endless support, assistance and guidance from Charlie Todd (MDIFW). Similarly, Bill Hanson (FPLE Maine Hydro) provided initial tree climbing and eaglet handling guidance early on, and continues to play a major role in these efforts every year. Many thanks are also owed to Steve Mierzykowski (USFWS), and Barry Mower (MDEP), and countless others. What have we learned from the study? That Maine’s eagle nests are not always easy to get to! I will follow-up later with a summary of some of the project findings and interesting tidbits… We’ve learned a LOT about Maine’s eagle population during this study, particularly their exposure to mercury contamination and impacts on the population. As with any good study, however, we’ve raised a lot more questions along the way, and we look forward to continuing to answer them in upcoming years!