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Biology 130: Methods in Field Biology

Field Technique:  Radio Telemetry

The following is excerpted from Wildlife Radio-telemetry Standards for Components of British Columbia's Biodiversity No. 5 (1998)

Wildlife radio-telemetry may be defined as the transmission of information from a transmitter on a free-ranging wild animal to a receiver. Wildlife-related telemetry is also known as radio tagging, radio-tracking or simply ‘tagging’ or ‘tracking’. Advances in the field of wildlife telemetry have made it possible to acquire detailed data on many aspects of wildlife biology, including habitat use, home range size, mortality and survivorship, and migration timing and routes. Since many wildlife species are secretive and difficult to observe, radiotelemetry has provided a valuable tool to learn more about their respective lifehistories. As a result, radio-telemetry studies are very common throughout the current wildlife literature (see Bibliography).

Despite its popularity, radio-telemetry is inappropriate under many circumstances. It is an expensive and time-consuming technique which has proven to be unsuitable for use in some species (due to the animal’s size or lifehistory traits). Despite the frequency with which radio collars and other transmitters are attached to research animals, surprisingly little is known about their effects on the behaviour and survivorship of the species in question. Certain First Nations groups strongly believe that collars and even ear tags influence behaviour and therefore actively oppose the use of these devices on game animals. The potential for modified behaviour and differential survival of radio tagged animals may introduce additional bias and error which could be reflected in study results. Quite clearly it can also be detrimental to the animal wearing the tag. The placement of a radio tag on an animal represents a commitment by the researcher, and there is the possibility that it is done at the expense of the animal it is placed on. Thus, transmitters should only be attached when project funding guarantees the ability to monitor a tagged animal for the life-span of the transmitter.

Transmitters must be attached in a manner which will minimize any effects on the study animal. Researchers should take extreme care when fitting harnesses and collars to ensure that they allow freedom of movement so that an animal’s movements are not hampered, but are not so loose-fitting as to increase the danger of entanglement. Additionally, they should exercise caution if using a new method of transmitter attachment and generally avoid any method which has been reported to cause adverse effects in the study species or similar species. Ideally, researchers should test attachment methods on captive animals before using them in the field. This also allows inexperienced researchers to become familiar with animal handling and transmitter attachment under controllable conditions. Zoos, game farms, falconers and wildlife rehabilitators are possible sources of captive animals on which to test transmitter attachments. (Note that permission should be obtained from the appropriate government agency before testing transmitters on releasable rehabilitation animals). More specifics of transmitter attachment are presented later in this manual.

Types of Transmitting Antennas

The two most common transmitting antenna are whip antennas and loop antennas:

Whip antennas:

Most frequently used. Omni-directional. Usually stainless steel with a Teflon coating. Must be light, strong, and generally withstand a tremendous amount of flexing.


  • Produce more uniform signal over a greater distance than do loop antennas.

  • Potentially subject to breakage through metal fatigue or corrosion; however, whip antennas may be sandwiched between layers of a collar for protection when attached to rough species (e.g., bears).

Loop antennas:

With collars, tuned to radiate maximum signal at the exact neck circumference for which they are constructed (±5%).

Pros/Cons: Signal does not travel as far as whip antenna.

  • Useful for species which would chew/pull whip antenna.

  • Wire loop may serve as both collar and antenna.

Rules for wildlife transmitter attachment

  1. Always carry a spare tag, to replace a dying tag or to test receiving equipment.

  2. Routinely record the signal pulse rate of tags, to detect the slowing which precedes cell failure.

  3. Tag more than one animal in a social group, in case one tag fails.

  4. Treat all animals with the utmost respect.

  5. Use the smallest possible transmitter package when instrumenting any animal. Although it may vary for some species, generally no tag should exceed 5% of an animal’s body weight. For some animals, such as bats, 4% may be a more appropriate proportion.

  6. Transmitter packages which are placed on animals which are dependent on cryptic coloration should be as inconspicuous as possible.

  7. Transmitters and their attachments should be tested on captive animals before they are tested on free-ranging animals. Test animals should be the same sex and age as the intended wild animals, and researchers must anticipate potential difficulties due to changes in an animal’s size, morphology, or behaviour over the course of its life.

  8. Transmitters should always be tested both before and after attachment to ensure that they are working correctly and that the magnets have been removed.

  9. Allow several days or up to one week for newly instrumented animals to acclimate to a transmitter before collecting data about “normal” behaviour.

  10. Whenever possible, avoid instrumenting animals during their reproductive period, as many species appear to be particularly sensitive to disturbance at this time.

  11. Seriously reconsider placing a transmitter on any animal that appears to be in poor body condition or impaired in some other way, unless it is particularly meaningful to the study to follow that specific individual. Recaptured animals showing adverse effects from transmitters should not be retagged. Researchers should not sacrifice the individual for the sake of a larger sample size

Once transmitter attachment is complete, the animal should be carefully observed before release. Short-term behaviours such as scratching at the collar or attempting to shake off a tag will generally cease when the animal becomes accustomed to carrying the transmitter. These behaviours should be distinguished from more serious effects such as improper balance, impeded movement or shifting harnesses which will require intervention. It is an unfortunate reality that many of these problems and behaviours will not be apparent or manifest until the animal is actually released and is difficult to recapture. This only serves to emphasize the importance of thorough research, preparation and testing beforehand.


Protocols - Materials

Collars are the most common form of transmitter attachment for mammals. Collars should be made of materials which :

  • are durable;

  • are comfortable and safe for the animal;

  • can withstand extreme environmental conditions;

  • do not absorb moisture; and

  • maintain their flexibility in low temperatures (Burger 1989c).

Common collar materials are butyl belting, urethane belting, flat nylon webbing, tubular materials, metal ball-chains, PVC plastic, brass or copper wire and cable ties.

Protocols - Fitting

The transmitter package may be situated either under the animal’s neck or on top of it. Collars must be properly fitted for the comfort and safety of the animal. A collar should fit snugly to prevent it coming off or chafing the animal as it moves, but it must also be sufficiently loose as to be comfortable and not interfere with swallowing or panting. To reduce the risk of chafing on the neck, Wildlife Radio-telemetry August 6, 1998 17 collars should generally be fastened at the side, with any metal fittings covered or smoothed on the inside surface of the collar. Neck circumference will vary according to species, age, sex and sometimes the season. Transmitter manufacturers usually have records of collar sizes previously used for various species.

Collar thickness and width are important considerations. Width of the collar will affect how the transmitter sits on the animal’s neck. Some researchers prefer narrower collars because there is less surface area in contact with the animal. Others prefer wider collars for better weight distribution (Burger 1989c). One of the most important considerations should be the possibility of the collar getting caught up in vegetation. This is a particularly important consideration with small mammals (especially those that burrow).

Expandable collars and harnesses are mandatory in those cases where it is necessary to allow for growth in young animals or for species which undergo neck swelling (e.g., male ungulates in rut) (Hölzenbein 1992). Braided nylon over surgical tubing and nylon web with elastic folds are offered as expandable collars by one company. Expandable collars should not be used unless they are well tested, as poorly designed collars can be very problematic. In the past, certain collars have stretched prematurely as a result of social interactions or behaviours such as neck rubbing. As a result, there is always the possibility of transmitter loss, icing up in winter, or of the collar becoming snagged by branches or even the animal’s own legs.

Protocols - Removal & Recovery

Breakaway or “rot-away” collars are strongly recommended in cases where the researcher does not intend to recapture the animal and remove the collar. Breakaway collars or harnesses incorporate a link of material which is designed to break away and allow the transmitter to drop off after a pre-determined interval. Breakaway links should be environmentally degradable material or electronic links controlled by timers or radio receivers. Environmentally degradable materials which have been used for this purpose include cotton thread and sections of cotton fire hose or cotton spacers on large mammal collars (Karl and Clout 1987; Hellgren et al. 1988). These weak links may also function to break and free the animal if the collar/harness is snagged on a branch. However, it is important to consider that the breakaway collar or harness does not impair the movement or activities of the animal during the period in which it is being shed. For example, a breakaway bird body harness could easily impair wing movement as it is lost and result in mortality. Radio and timer-controlled breakaways may be jammed by freezing or dirt, and also add to the size, weight and complexity of the transmitter package.

Where appropriate, it is recommended that collars and harnesses be marked in order to enhance their visibility. Paint or non-metallic reflective materials may be sewn or glued to collars and harnesses; however, this is likely not appropriate for cryptic species. Metallic tape or foils should not be used as they will detune the transmitting antenna. Adhesive tapes should also not be used as they are not very durable and may foul fur or feathers. For game species or urban studies it may also be helpful to mark a contact phone number on the collar. Colour-coded collars are also available from telemetry equipment manufacturers.

Other Common Methods of Attachment

As it is not possible to establish detailed protocols for all methods of attachment, this section presents a selection of commonly used methods and some key considerations surrounding successful deployment.

Tail Mounts

Description: Tail Mounts are attached to the tail feathers of a bird. In different studies, they have been glued (including wax) or sewn to a bird’s rectrices or attached to them with cable ties or alligator clips (Bray and Corner 1972; Kenward 1987).

Key Considerations: Transmitters attached to tail feathers are lost when the bird moults. Rectrices should be handled gently while the transmitter is being attached as stresses to the base of the feather may result in its being moulted prematurely. Considerations should be made with respect to the organism’s life history. Depending on how an animal uses its tail, a tail mount may be inappropriate (e.g., woodpeckers - antenna may snag in bark of a tree).

Implantable Transmitters

Implantable transmitters are best suited for species:

  • whose necks are not well-defined (e.g., snakes),

  • whose heads are smaller than their necks (e.g., male polar bears),

  • which might be impeded by an external transmitter (esp. burrowing animals),

  • which are sensitive to external attachment (i.e. amphibians),

  • which are young and expected to grow.

They are also used for certain biotelemetry applications (especially body temperature). Implants are sealed with neutral (biologically inert) epoxy, resin, or wax, and implanted into the body cavity or under the skin. The antenna may be left external to the body, implanted under the skin or it may be contained entirely within the implant unit.

Key Considerations: Despite the initially invasive nature of this technique, one of the key advantages of implants is that they may be much less irritating (if implanted correctly) to the animal than an external tag. Implanted transmitters have a fairly limited range. Those with an implanted antenna will have an even shorter range, but will be less subject to damage or infection than transmitters with external antennas. Transmitters are also expensive to implant as they generally require that researchers employ a qualified veterinarian. Animals may also need to be Wildlife Radio-telemetry August 6, 1998 19 held for a protracted period in order to recover from the effects of a general anaesthetic.

Backpack Modules

Description: Backpack modules are attached to the animal by a harness, which is often run through tubular passageways on the transmitter pack (Nicholls and Warner 1968; Jackson et al. 1985; Ward and Flint 1995). Harnesses may be made from soft Teflon ribbon, plastic-coated wire, metal beads, plastic beads, surgical tubing or polyester soft stretch elastic.

Key Considerations: The style of harness used depends on the study species, and it is generally necessary to test a harness style on captive specimens before using it in the field. Some manufacturers offer ready-made harnesses to fit the more common species. Elastic harnesses will eventually degrade and free the animal from the transmitter and biodegradeable sutures can be used to release harnesses from aquatic animals. Kenward (1987) states that it is best to avoid the use of harnesses for a species that can be tagged any other way, as the animal may potentially wear the harness for the duration of its life, and even the best-fitting harnesses may eventually snag.

Adhesive Transmitters

Description: Adhesive transmitters may be glued onto an animal’s body, often its back, with cyanoacrylate glue, false eyelash cement, surgical bond (skin cement) or other glue-like substances. Titan Seabird Glue is used by one researcher to attach tags to Dunlin (P. Shepherd, Simon Fraser Univ., Burnaby, B.C., pers. comm.). In birds, the area is usually prepared by trimming feathers to 2 to 5 mm in length. Carapace mounts are typical with turtles while other reptiles have had tags taped to their tales. Mammals can have them glued directly to the fur (e.g., bats, voles) or sometimes the fur is removed before attachment. Typically the whip antenna runs dorsally and caudally to the long axis of the body.

Key Considerations: Depending on the type of adhesive used, the tag will generally detach itself. Preparation of an attachment site on the animal may require clipping/shaving which may induce additional stress and potential physiologic problems, such as interference with thermoregulation or flight. Aggressive grooming of adhesive transmitters may shorten their active life further than limits imposed by the power supply.

A few YouTube videos showing some aspects of how radiotransmitters are attached to wildlife:



Necklace Packs

Description: Necklace packs are often used on upland game birds. These packages simply hang down on the breast of the bird, supported by light flexible cable or cord around its neck. The cord is run through a sleeve to protect the bird’s neck.

Key Considerations: This system is probably the easiest and quickest to mount, resulting in shorter bird-handling time. Necklace must be long enough to allow the animal to swallow large food items without choking. 3.3.6 Eartag Transmitters Description Although originally designed for use on polar bears, eartag transmitters have since been used on other species of bear and ungulates (Telonics). They are particularly favoured for large animals with changing neck girth (e.g., juveniles, male cervids). A round design and foreshortened antenna allow the transmitter to rotate freely while remaining in place. Key Considerations This type of transmitter can range over 3 km given ideal conditions (flat landscape, open vegetation, dry environment). Long antennas should be avoided as they can annoy the animal.


Descriptions: Numerous other techniques have been used to attach transmitters to animals. For example, meshwork vests with transmitters attached have been used on birds (Lawson et al. 1976). Numerous other examples can be found in Chapter 7.

Key Considerations: If an attachment design is new, it is critical to test it adequately, preferably on animals in enclosures before conducting a study in the wild. It is important to keep in mind, however, that a captive animal may not have the same physical demands as a free-ranging one. For example, a captive raptor may tolerate a certain attachment technique, but a wild bird will have to fly and hunt with it.



The function of a receiver is to receive the signal picked up by the antenna (to which it is connected by a coaxial cable), amplify it, and make it audible to the user. Receivers are available in a variety of sizes, weights and prices from a number of national and international suppliers. Study needs will determine whether data collection is best done manually by field personnel or whether an automated receiving station should be set up. Receivers are powered by replaceable and/or rechargeable batteries, and may also be equipped with a cigarette-lighter adapter for connecting to a vehicle’s electrical system. Some models are equipped with scanners which may be programmed to switch between a number of different frequencies; this is ideal for studies with a number of animals which tend to wander. Strip-chart recorders or data loggers may also be incorporated into a receiving system, and are particularly useful for automated receiving stations.

(A Telonics receiver. You can choose the channel corresponding to the radiotransmitter signal. Then you can fine tune the signal.)

Protocols - Handling

Receivers may be damaged by static electricity from clothing or car seats and by radiated power from voice communication systems (Crow 1988). To prevent this damage:

  • clothing and vehicle seats should be treated with antistatic fabric softener (especially during cold weather);

  • receivers should be turned off and the antenna disconnected when getting in and out of vehicles and when using voice communication systems; and

  • roof-mounted antennas should be separated from transceiver antennas.

It is also worthwhile to note that receivers are sensitive to moisture. This is an important consideration when try to locate animals in the rain.

It can be useful to adjust a receiver up or down in order to identify the best or most functional frequency for a given transmitter. It is not uncommon for a transmitter’s best frequency to be slightly different from the one identified by the manufacturer. As well, a transmitter’s frequencies may drift slightly.

How to use a receiver:




Receiver antennas may be hand-held or mounted on a vehicle roof, aircraft or boat. A Yagi antenna is a directional ‘gain’ type antenna which uses a number of parasitic directors in front of the ‘driven’ element (the one connected to the coaxial cable) and a reflector behind the driven element in a defined mathematical relationship (Jones 1990). Directional antennas such as Yagis or ‘H’ antennas concentrate the radiated energy to the front of the antenna. Minor lobes to the sides and rear are also produced.

Antenna beam width refers to the radial distance between the angles at which an antenna is held in which an audible signal is received (the ‘directionality’ of the antenna). The greater the number of elements, the smaller the beam width. For example, a 3-element Yagi antenna has a beam width of 60o in the horizontal orientation, and a 2-element H antenna has a beamwidth of 100 degrees in the horizontal orientation. Both antennas have wider beam widths in the vertical orientation (Burger 1991).


Regardless of the type of antenna, the elements must remain straight and parallel to one another to ensure maximum receiving efficiency. Antennas are tuned to a particular frequency, and antenna elements are only interchangeable if they are the same length as the original elements, and are interchanged between antennas of the same frequency range. Maintain antenna elements in perfect alignment; badly damaged elements are likely beyond repair.

Types of Receiving Antennas


The most commonly used hand-held antennas are the Yagi and the ‘H’ antennas. Hand-held Yagis have 2 to 5 elements. Each additional director element increases the distance from which the antenna can pick up a signal.

receiver and yagis
(The image shows a Telonics receiver and 2 types of radio antennaes. The one on the left is a H-antennae, the one on the right is a 3 element yagi. Both can be made more portable. You can unscrew the elements of the H-antennae and you can fold down the elements of the 3 element yagi. Both attach to the receiver with a coaxial cable. Note: the weakest point of the set-up is the coaxial cable so it is always advisable to have at least one extra cable with you in the field and to test the cable regularly. If it is broken you won't get a signal and you won't know it is broken unless you test it in a situation where you know you are supposed to get a signal.)

Loop antennas are small, hand-held antennas which are useful for close-in tracking of 1 km or less.

Boat or Vehicle-mounted Antennas

Large directional antennas with 5, 8 or 14 elements are usually used as vehicle mounted antennas or at fixed sites. Omni-directional (bipole) antennas may be mounted on a vehicle and used to determine the general vicinity of an animal. A precise location can then be determined with a directional antenna.

Aircraft-mounted Antennas

Both Yagi and H antennas have been used for relocating animals from the air. Antennas are mounted on fixed-wing aircraft with brackets designed to fit struts on commonly used types of aircraft. The operator uses a switch box to listen through the left or the right antenna or both to determine the direction of the incoming signal. The receiving system can be connected to the aircraft intercom system so everyone in the aircraft can hear the transmitter. Brackets to mount an antenna on a helicopter skid are also available (Telonics).

Brackets must be chosen carefully as strut sizes may vary within the same model of aircraft. The antennas should be centered on the struts with the tips of the antennas facing fore and aft, with the front of the antenna facing out toward the wing-tip and slightly downward (about 30 degrees below the horizontal axis of the wing, Jones 1988). Some researchers recommend orienting the antennae at 90 degrees to each other. Duct tape is often used to attach antenna coaxial cables along the outside of the wing and through windows or vents into the cabin. Antenna / coaxial cable attachments should also be secured or taped as they can become loosened by constant vibration and jeopardize the results of the flight. All equipment attached to aircraft is governed under aviation law and requirements may vary according to ownership, use and location. Researchers must ensure that their equipment and their means of attachment comply with the appropriate aviation standards.

Fixed-Site Receiving Stations

Automatic direction finding systems incorporate a rotating antenna, a fluxgate compass and a receiver/datalogger that automatically determines and stores bearing and signal strength information on a particular animal for downloading. Very large antennas with many elements may be installed at a fixed receiving station. Antennas at fixed sites are subject to lightning strikes and therefore should be well-grounded. Fixed receiving stations require some prior knowledge of the animal’s range to ensure the best placement.

The accuracy of a radio-location varies with habitat type and may result in biased estimates of observed habitat use. A common source of error is signal bounce. Signal bounce occurs most frequently in mountainous terrain where a signal is deflected by a mountain, resulting in potential errors of many kilometres. The most effective way to overcome signal bounce during ground tracking is to take many bearings from several different places. When all signals appear to be coming from the same point then there is a good chance that the animal has been located correctly. However, if the signals are coming from a number of different points then signal bounce is likely still occurring (White and Garrott 1990).

Visual observations of radio-located animals provide the best confirmation of the accuracy of the relocation data. For large animals, a reasonable proportion of locations should be confirmed by direct visual observations (some biologists use >30% as a general rule; however, this may not be practical in all cases). In new study areas or with species which cannot be observed on a regular basis, it is strongly recommended that triangulation be used with an assessment of aerial fixes made using collars placed in known locations. Such trials can test the consistency and accuracy of triangulation using various personnel and methods under various environmental conditions. Results of the trials can be used to identify problems (e.g., signal bounce) and ensure that methods are adjusted to reliably obtain accurate radio locations.

When relocating wildlife in the field, most users judge the angle over which the signal sounds loudest, determine a bearing by mentally bisecting that angle, and follow the bearing to move closer to the signal. The process is repeated until the animal can be seen or its location can be inferred. The latter may be accomplished by circling the signal to determine a bounded area in which the focal animal must occur, tracking the animal to an obvious habitat or landscape feature, or by sandwiching the animal between the receiver and an apparent obstacle.

Alternatively, if the researcher wishes to avoid disturbing the animal, or if locations must be determined at night, the process of triangulation may be used. This requires finding the intersection of two or more bearings to determine one location. An error polygon can be calculated around the point estimate, resulting in a measure of precision equivalent to the area of the polygon. The size and shape of the error polygon is determined by:

  1. the accuracy of the directional antennae;

  2. the distance between the two receiving points;

  3. the distance of the transmitter from the receiving points; and

  4. the angle of the transmitter from the receiving points.

The most accurate estimate of an animal’s location is obtained by receiving fixes that are closest to the animal and at 90 degrees from each other. To reduce the size of the error polygon, three bearings can be taken and the animal’s location estimated from the centre of the intersections. The error polygon formed by three radio bearing lines should be small enough to accurately place the animal in a single habitat polygon. If the location is near an edge, additional bearings should be obtained to accurately locate the animal on the map.

Finding the direction of the transmitter:

To determine the direction of the transmitter you have to know the signal profile. For a single antennae the signal profile is a plateau. This means there is no "loudest" signal in the direction of the transmitter. Rather you have to find the edges of the plateau (the signal drop off points), then choose the direction that is the mid-point between those drop off points.

signal profile single antennae

There is another way to get a more accurate direction if you have 2 antennaes set up in parallel. This creates interferrence in the signal that results in a "null-point." You can't do this with hand held antennaes, they have to be mounted on a fixed T-bar to keep the antennaes parallel. The direction of the transmitter is where the signal goes from loud to nothing.

null point signal profile

Triangulating with radiotelemtry:



Where possible, standard telemetry base points should be established, marked and numbered by personnel experienced in use of radio-telemetry equipment. New observers should be familiarized with the base points and standard triangulation procedures by an experienced person. Triangulation of animals which are moving will produce large polygons (less accurate locations). For this reason, it is difficult to accurately determine locations of fast-moving nocturnal wildlife such as owls. If triangulation is used to determine wildlife positions, error measures should be calculated and reported along with the study results (Springer 1979; Saltz and Alkon 1985; Schmutz and White 1990; Saltz 1994). White and Garrott (1990) provide a useful compilation of error calculations for telemetry.

Determining declination: NOAA website

Guidelines for ground-based telemetry:

  1. Ensure that you use an antenna which is matched to the frequency transmitted.

  2. As a general rule, the antenna elements should be oriented in the same direction as the transmitter antenna (i.e., when relocating a caribou wearing a radio-collar with a vertically-oriented whip antenna, the receiving elements should also be held vertically).

  3. Hold antennas as high as possible or mount them on poles. Keep antennas at least 2 m away from all other objects, especially those which are large and metal objects, as these will cause detuning of the antenna.

  4. Make use of null signals as well as peak signals to determine the direction to a transmitter. Using a 2-element antenna, the signal should be weakest when the tips of the elements point directly at the transmitter.

  5. Make use of hills and other places of high elevation from which to receive signals.

  6. Know your study area. Whenever possible take bearings through the flattest terrain with the least vegetation.

  7. Take repeat bearings over a short time period, especially if the animal is active.

  8. Get as close to the animal as possible. Attempt to confirm locations with direct observations.

  9. Avoid sources of interference.

  10. Take as many bearings as practical.

Condor Information:

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Revised 26 January, 2015
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