Tracking Golden Eagles in San Diego County
By Amber Ward

Dave Bittner uses a smaller field tracking device to follow eagles. PHOTOS BY AMBER WARD

The Wildlife Research Institute, Inc. of Ramona, Calif., is protecting Golden Eagles through research. Using satelite telemetry and solar-powered VHF transmitters, WRI is tracking the movements of San Diego County Golden Eagles. The status and fate of this native Californian species is important because Golden Eagles are an "umbrella species," their health is indicative of the health of their environment and other species. With the use of telemetry, WRI is working to preserve Golden Eagles and their habitats, as well as determining the effects of wildfire on eagles and other species.

"With the use of telemetry you can easily track eagles," says Dave Bittner, WRI Wildlife Biologist. Mountain Lions are also an "umbrella" or indicator species, but it is much more costly to track these animals than it is an eagle. The light-weight, solar-powered VHF transmitters that are placed on an eagle's wing are very inexpensive compared to the battery powered collars that a lion would wear.

Bittner records data in his file.

WRI has an established team of Golden Eagle biologists, bioclimbers and volunteer observers that have been active in San Diego County since 1988. Using over 130 years of historic information, yearly field surveys, and current research, WRI has been able to document the entire eagle population both past and present.

In the 1800s and early 1900s egg collecting and egg trading was a very prestigious hobby. Oologists (egg collectors) in San Diego County kept detailed records of what and where eggs were being collected. The records of these collectors have provided early documentation of Golden Eagle nest sites, and many of these sites, or Golden Eagle territories, are still being used by the eagle population today.

James Dixon, an early oologist documented 28 Golden Eagle teritories in San Diego County between 1895 and 1936. Of those original 28 territories, only nine exist today, the other 19 of the territories no longer exist because of human encroachment. An eagle's territory ranges from 20 to 100 square-miles depending on the area's food supply.

This 20-foot antenna can locate eagles for miles.

Once, there were 104 pairs of nesting adult eagles in San Diego County, today there are only 46. Because of human encroachment, and the damage and destruction of suitable Golden Eagle territory, the county cannot support more eagles. "Most of the eagles lost have been in the western portions of the county," notes Bittner, "and even if a territory opens up because of a death, it can take up to 25 years before a pair of eagles will successfully breed again in the area."

In 2005, WRI was awarded a total of $83,000 in grant money, with $70,000 coming from the United States Forest Service, in support of their Golden Eagle population studies. In both the 2003 and 2007 wildfires in San Diego County, crucial Golden Eagle habitat was affected by the fires. The current goal of WRI's research in conjunction with USFS is to understand how fire affects Golden Eagles and their habitat. When are Golden Eagles using the burn areas, and when is their most productive time after a wildfire has moved through their territory are questions they are trying to answer through eagle tracking.

"Fire is not a problem for Golden Eagles, in fact its a good thing," says Bittner, who is currently working on the project. "Eagles hunt in open grass and cannot hunt in dense forest or in heavy chapparrel, the burn area becomes the ideal hunting space."

Between 2006 and 2007, 36 Golden Eagles have been tagged with the solar cell transmitters in San Diego County alone. The solar cell transmitters cost about $200 each, while the more expensive satellite transmitters cost nearly $4,000, and another $1,000 a year just to maintain. Traditional telemetry came about in the 1970s and has proven to be very effective in monitoring the movements of a variety of birds.

Every Saturday, volunteer biologists go to local mountain peaks to try and determine the location of the tagged eagles that register on the scanner. Otay Mountain, Cuyamaca Peak, Palomar Mountain, Mt. Woodson and Black Mountain are all points where biologists attempt to determine eagle locations. They listen for signals that are picked-up by their telemetry antennas, and once a signal is heard its approximate location is recorded using a compass heading.

A direct line of sight is needed for a strong signal, and if the bird is in flight, then the signal strength will vary. If the eagle is down in a valley, then the signal be weak if available at all. The goal is for two biologists to pick-up the same signal from two different points at the same time. Using both compass headings, an intercept can be determined, identifying exactly where the eagle can be located.

This study will provide an accurate record of Golden Eagle activity in burn areas. With use of this data, evidence of eagle activity in burn areas will further provide evidence that brush management is an effective way to secure Golden Eagle habitat. In fact, it is not only important in habitat management for eagles, but also for humans. Eagles and humans both require a dynamic environment in order to thrive.

If you encounter an eagle with an orange tag, or two tags on its wings, it is important to contact WRI with information about the exact location and condition of the bird. The Wildlife Research Institute, Inc. can be reached at 760-789-3992, or e-mail d-bittner@wildliferesearch.org.