Here are answers to some frequently asked questions about the type of transmitters that some of our former eagle patients are wearing.
Q: Why do you put transmitters on some eagles prior to release?
A: The eagles wearing GPS transmitters are part of a larger, ongoing research study that monitors eagle movements. This study looks at the data received from Bald Eagles to determine the range and behavior of the eagles in Virginia’s coastal plain. Migratory behavior is studied as biologists are able to see how far Bald Eagles move in the winter season, and the data will play an important role in modeling how these birds use airspace. By looking at heights at which the eagles fly, average distances, and other specifics, biologists are able to relate this eagle behavior to real-life issues, such as airstrike data. During the past few years, VDGIF Biologist Jeff Cooper has fitted many Bald Eagles with GPS transmitters.
For the Wildlife Center, having access to the data on tracked eagles is a fantastic opportunity for additional post-release studies of our rehabilitated raptors. There have been very few studies done in this area. We look forward to watching the movements and behaviors of birds that have been treated at our facility.
Q: How does the transmitter work?
A: These types of transmitters use satellites to record GPS information. By using a cellular network to transmit data, the transmitters are able to provide GPS points every 15 minutes. During one seven-day period each month, the satellites will record GPS information every 30 seconds, so that researchers can create and utilize three-dimensional maps for their research. Data is temporarily unavailable when the birds fitted with these units fly out of cell phone range, but all information is stored and readily available once the bird is back in range.
Q: How are the transmitters attached to the birds?
A: GPS transmitters are fitted on eagles with Teflon straps – similar to how a human would wear a backpack.
Q: Won’t the transmitter weigh the eagles down?
A: No, the transmitters are very light and weigh less than 80 grams. In general, when transmitters are fitted onto eagles and other large birds, it is recommended that the weight of the backpack not exceed three percent of the bird’s body weight. For these eagles, the transmitters are less than two percent of body weights. In 2018, a new generation of transmitters were put into use; these are even lighter.
Q: How long will the transmitter last?
A: This type of transmitter relies on lithium batteries but the units are also adapted with solar chargers. The biologists estimate these particular transmitters will last between one to two years. They are still learning about the battery life in this generation of transmitters due to the different data parameters; the life of the battery should be shorter than the battery life of NX’s transmitter since her “data dump” was set to every other day, and her data points were recorded every 15 minutes. These are rough estimates; the prediction was for NX’s battery to last about two years, and as of August 2015 it has been four years and she’s still checking in!
Q: Will the transmitter fall off after the battery expires?
A: These transmitters are permanent; they will either stay on for the lifespan of each bird or will be taken off if the battery is dead and the birds are ever caught.
When the GPS transmitter is placed on a bird, the vital part of the fitting that secures the backpack in place is the piece of line that holds the two straps together. While cotton string [which can degrade] can be used, the biologists working on this study prefer working with an alternative material. West Virginia University Professor and Cellular Tracking Technology biologist Dr. Todd Katzner prefers using a stronger woven line that won’t break down. Dr. Katzner, who has been attaching transmitters to birds for more than 15 years, feels that this option is safer for the birds in the long run because it is unlikely to partially fall off, leaving a dangling strap that could entangle the bird.
Generally speaking, the tear-away variety are used when researchers want to recover the transmitters.
Q: How often will we be able to see where the eagle is?
A: Depending on the parameters of the study, the unit may provide data every day, or every other day. If the battery voltage remains low after a period of time without adequate solar charge, the duty cycle may be adjusted by tracking technicians to a less frequent check-in to preserve the life of the battery. Transmitting the data also requires that the bird be “in-range” at the check-in time.
For example, if the transmitter is set for a data download at 3:00 pm EST – we will receive that information at 3:00 p.m. on check-in days. However, if the bird flies out of cell phone range on a transmitting day even if only from 2:55 p.m. to 3:05 p.m. – the data will not be received that day. The data will be stored, however, so that if the eagle is back in range the next day, all movements will be tracked and recorded.
Because the Wildlife Center staff logs into the tracking system to see data, look at maps, and create updates on multiple eagles, it's likely that updates will be provided once a week on each bird.
Q: Will the eagles fly long distances?
A: We’ll have to wait and see! In general, a Bald Eagle’s daily activity depends on the age of the bird and the season. According to the Birds of North America online, some studies suggest that immature eagles spend less that two percent of their time flying and eating; more than half of their time is spent roosting, and about a third of their time is spent perching. Adults typically spend a little more time in flight, but still spend a majority of their time perching and roosting.
Some eagles choose to stay in Virginia year-round; NX, for example, remained in or very near Virginia in her five-year tracking period. We've seen other eagles travel much farther -- NC99 flew from Virginia to South Carolina for several months before he returned to central Virginia. After months of hanging out along the Potomac River, MN18 flew to Canada! Many other Bald Eagles travel around the Eastern Seaboard. We’ll have to tune in to see the adventures of these tracked eagles.