Caribou Migration

Confused about Collars?
Conventional and Satellite Collars

Caribou and many other wild animals of interest are studied in their habitats using collars with transmitters. Wondering about the difference between conventional and satellite collars? We know they are both used to help locate animals in the herd, and we know they both require some kind of battery to send a signal. Here are some more details to help clear the confusion.

Conventional Collars Use Radio Signals
Conventional radio collars, sometimes called VHF transmitters, emit radio signals at frequencies between 132-174 MHz, and can be heard from about 10 + miles away under ideal conditions. The maximum possible distance that the collar can be heard depends on a number of factors:

elevation of receiver - airplane/helicopter
strength of signal
age and type of collar

Fran Mauer and plane with radio antenna under each wing

Radio signals transmit by line of sight, so if the transmitter (collared animal) is in steep mountainous terrain, it might not be heard until you were directly overhead, whereas a collared animal out on the coastal plain would probably be heard from a quite a long distance away. Each collar is set up with a different radio frequency. Biologists use special antennae tuned for the frequencies of the collars they are listening for, which are mounted under the wings on airplanes when tracking.
When tracking for conventional collars, the plane usually flies transects across the range (ie. back and forth following lines of latitude or longitude) until a collar's signal is heard. If the collar signal sounds close by, the plane may break off the path to obtain a specific location.

Satellite Collars have Stronger Signals

Courtesy US Fish and Wildlife Service

A satellite collar can send a signal in the dark of night and even in the middle of a blizzard. two satellites that orbit the earth from North to South Pole 14 times each day. These satellites orbit the earth over 500 miles up (over 800 km). They scan for the collars signal and with the aid of computers onboard, locate the caribou within 1000 meters of accuracy. The collars on the Porcupine Caribou are programed to transmit a signal for 8 hours one day a week. At this rate, the collars should have battery power for 18 months.
We use the satellite collars to predict where we might find the majority of the caribou. Even though there are only 8 satellite collars on this herd, the satellite collared cows seem to have dispersed themselves randomly enough so that the areas where the major concentrations of caribou are are indicated.
The satellite collars have two activity counters, short and long term - one tells us the number of seconds within the past minute that the caribou has moved immediately before that particular transmission, and the other tells us the number of seconds within the previous hour that the caribou has moved. Sometimes caribou don't move much for weeks because of their grazing habits, however this kind of data can act as mortality sensors as we found out in February, 2002 when Trudy's sensor revealed that she had died.

Collars' Costs Compared
A satellite collar is really expensive. One collar costs about $2,700! Conventional collars are about one-tenth the cost of satellite collars, so you can obviously buy a lot more of them. Researchers try to maintain 100 conventional collars on Porcupine Caribou. They use the radio collars to find the herd at various times of the year, including during spring migration, calving, post-calving and during post-calving aggregation (during census years), sometimes during fall migration, and for late winter composition counts and collaring.

Collars Help Researchers
Collar transmissions can help researchers get a good idea what is happening with the herd. They help determine answers to many population and behavior questions.
When Steve Arthur and his Alaska Department of Fish &Game associates locate the collared cows during calving and post-calving, they use the collared sample to extrapolate how many cows (overall within the herd) gave birth, and then how many of their calves survived the first month of life (which is when mortality rates are highest).
For example, in 2001, 70 radio-collared cows were located, of which 59 or 84% were pregnant or gave birth. At the end of June, 61% of those calves born were still alive. Steve would be able to fill in more details if you have further questions.

To learn more about Satellite tracking:

Satellites and Caribou