Radiotelemetry

The radiotelemetry program provides information that is impractical to obtain in any other way. The purpose of the study is to learn about the movements of the individual hawks, as well as hunting behavior, flight patterns, and roosting areas. The Golden Gate Raptor Observatory Radiotelemetry Program began in 1990, and was the first such study of hawk movements on the Pacific Coast. The most complete data have been obtained on juvenile red tail hawks and Cooper's hawks.

How it's done

When the radiotelemetry team is ready to begin tracking, the team assembles in the Marin headlands and waits until a hawk that meets the predetermined criteria is captured at one of the banding blinds. When a suitable hawk is captured, a small transmitter (5.5-6 g) is epoxied to the base of one of the tail feathers. Present regulations do not permit the attached weight to exceed 3 percent of the bird's weight. However, GGRO policy is to limit the weight to two percent of the hawk's weight. The wire antenna is about 12 inches long. It is manually sewn at intervals along the tail feather, and extends a few inches beyond the end of the tail feathers.

The transmitter emits signal pulses with a duration of about 1/50 second, 60-90 times per minute, at a frequency of 150.xxx MHz, where xxx denotes the digits giving the frequency for the particular transmitter. The batteries, which constitute the major weight of the transmitter, last about ten weeks.

The transmitter ideally stays on the hawk until the tail feather molts; however, several Coopers hawks have pulled the feather with the transmitter attached, thereby prematurely ending the ability to track them by radio. The goal is to study the natural behavior of the hawks, so the question as to whether or not the presence of the transmitter affects the actions of the Cooper's hawks is troublesome beyond the inconvenience of the early loss of tracking ability for some birds. This issue has risen frequently enough that the telemetry study on Cooper's hawks has been suspended until an alternative option for antenna attachment or design is developed.

After release of the bird, three tracking teams follow its path. Each tracking team is composed of one or more volunteers. Each team has a designated leader, and the leader of one of the teams is the chase leader, who coordinates and directs the others. Three vehicles are used; ideally they are four-wheel drive, which may be needed to navigate the roads that lead to some of the high points.

Communication between the tracking teams and with the home base is critical during a chase. It is accomplished primarily by three technologies:

While GGRO does not provide cell phones, many of the volunteers now have their own, and they provide an important additional communication means. However, in following a hawk, the teams are often led to remote areas where there may not be coverage by cell phones or pagers.

Typically, two teams are located on "high points" and one team follows the hawk. As the hawk moves from one area to another, the teams on high points move to new high points to maintain contact with the hawk, as directed by the chase leader.

The transmitted signal is detected by a receiver in conjunction with either an omnidirectional antenna mounted on the top a chase vehicle or a directional antenna, known as a Yagi antenna, which is used from a stationary point to determine the direction to the transmitter. Knowing the locations of each team and the direction to the transmitter, triangulation can be used to determine the location of the hawk. Readings are taken every 15 minutes on the quarter hour throughout the day insofar is practical, providing good location data so long as the signal can be detected.


Libby Rouan listens from the peak of Mt. Tamalpais for 'Ohlone' with the Yagi antenna.


The strength of the signal received depends upon many factors, including the attitude of the hawk (meaning its physical orientation, not its emotional reaction to the situation), intervening or adjacent terrain features, and noise from other radiosources. Successful measurements of direction from a mountaintop to a hawk in the air at ranges of greater than 30 miles are not unusual. On the other hand, one can be quite close to the hawk and be unable to receive a signal if the transmitter is near the ground or if the transmitting antenna is oriented end-on to the receiver.

Tracking begins each morning at first light and continues until dark, when a final set of measurements to locate the evening roost is taken. A log of all measurements and locations and events is maintained. Summaries are prepared each evening and the plans and strategy for the next day are then formulated. Common practice has been to spend the nights at inexpensive motels.

Trackers are asked to sign up for a stretch of about four days, although every attempt is made to meet individuals personal constraints. Personnel changeovers are made in the evening at whatever location the target hawk has led the chase.


John Moody tracking from the top of San Pablo Ridge.

Telemetry training

A morning-long training session is held on a weekend in September for the tracking sessions which have been conducted in October and November. The training includes a review of the procedures and familiarization with the equipment. In the field, apprentices are paired with experienced trackers, who provide "on the job training."


Jeff Boissier tracking Shirley, a juvenile red-shouldered hawk,
from the peak of Mt. St. Helena (Napa Valley) in Autumn of 2000.

Commentary

I have found participation in this program to be a great experience. There was and is still a lot to learn about hawk behavior and the methods for tracking. The days are long, full and challenging. The people involved are highly motivated and eager to pass on information and coach beginners. For a detailed description of how three days of tracking went in November 1997 (probably more than you want to read), go to the Radiotelemetry Chronicle.

 


Linda Yuen tracking from just below the peak of Mt. Tamilpias.

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