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      Beat Naef-Daenzer ran a series of comprehensive tials to test the effects of radiotransmitters on a variety of tit species in the 1990s.  He concluded that even the smallest tits could carry up to 5% of their weight (Naef-Daenzer 1993 & 1994).  A 5% threshhold has become the general rule for attaching radio transmitters to birds.  Although radiotelemetry can provide very important data on bird behaviour it presents a great challenge to researchers who must balance this weight restriction against battery life and seek a non-invasive attachment technique.

      In my research I use radiotelemetry to track female Black-capped Chickadees (average weight 10.8 grams) for a few days during the females' fertile period.  Based on good advice from other researchers I found chickadee telemetry can be a relatively easy and rewarding technique.  This page is intended to pass along my experiences with chickadee telemetry to other researchers interested in radiotracking small passerines.

Female black-capped chickadee with LB-2 transmitter

I use Holohil LB-2 transmitters, which weigh 0.5 grams.  I do not use attachment tubes to minimize transmitter weight.  The people at Holohil are friendly and knowledgable and they provide a good product, although there is often a backlog of orders because their product is in high demand.  I recommend Holohil very highly.

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I use a figure-eight style leg harness (Rappole & Tipton 1991) to attach LB-2 transmitters to chickadees.  This harness works as a backpack where the "loops" of the figure-eight wrap around the bird's legs while the transmitter sits across the bird's back.  I know this harness has been used succesfully on White-crowned Sparrows (Zonotrichia leucophrys) and Hooded Warblers (Wilsonia citrina) and with moderate success on Least Flycatchers (Empidonax minimus). 

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How to make the figure-eight harness

(Follow the diagram at right)

1.)  Cut a short piece of beading cord elastic.  This stretchy elastic is available at sewing shops.  Use the thin stuff.  (The perfect thickness of beading cord elastic is what they use to make candy necklaces.)

2.)  Tie one end to the middle using a loose granny knot to create one loop.

3.)  Tie the other end to the middle (but inside the first loop so the knots can pull on each other) to create a second loop, and thus a figure eight.

4.)  While the knots are still loose, adjust the lengths of the loops so that the loops are the exact same size.  Then tighten the knots.  See below for information on harness size specific to Black-capped Cickadees.

5.)  Strech the figure eight over two nails or pegs.  Don't over-stretch the harness, just stretch it to the tightness it would have if it were fitted on a bird.  Cover the entire knot with a dab of 5-minute epoxy.  Let it dry overnight while suspended around the pegs.

6.)  After the epoxy is solid, trim the excess beading cord elastic.  Using a pocket knife and sandpaper, scrape away all excess epoxy.  Try to get a super-thin layer of epoxy around the knot. Weigh the harness when you're done to find out if you've shaved off enough epoxy.  Use very fine-grain sandpaper to smooth the side of the epoxy-knot that will be next to the bird's skin.  Keep the other side of the epoxy-knot rough. 

7.)  Glue the transmitter to the rough side of the epoxy-knot using super-glue.  You can do this before you activate the transmitter, but be careful not to glue the leads together.  Store the harness in its relaxed state (i.e. not on the pegs). I think the two most important steps in the above technique are (i.) using thin beading cord elastic and (ii.) while the glue is drying, stretching the harness as though it were fitted on a bird. Special thanks to Scott MacDougall-Shackleton for telling me about these two important steps.

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Despite what you may have heard, size does matter.  Here are the measurements that I found worked best for Black-capped Chickadees. With the figure eight laying relaxed on a table: 3.0 cm across the whole figure eight. Stretching the figure eight between two pegs just enough so that it doesn't sag: ~1.5 cm across the inside of each loop, ~3.5 cm across the whole figure eight.  Stretching the figure eight so much that you think you're going to stretch-out the elastic: 6.5 cm across the whole figure eight.

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Attaching the harness to birds

    The beauty of the figure-eight leg harness is that you can put it on and take it off easily and you can re-use the same transmitter/harness on successive subjects.  Once you get some practice, you can put a harness on a bird in a minute or two and you don't have to cut any feathers or wait for glue to dry in the field.
    While holding the bird upside down in the banding grip, stick one of the bird's legs through a loop.  (Make sure that the antenna of the transmitter/harness will point down the bird's tail!)  Using a blunt-end (I use a really dull pencil), gently ease the first loop up on the bird's first leg as far as it can go.  Once you get the first loop up high on the bird's leg, you can apply a little bit of pressure to keep this first loop in place by pulling the other loop around the bird's back.  As you are positioning the transmitter/harness to insert the bird's second leg, keep the transmitter as high on the bird's back as you can; this will prevent the transmitter from falling off by sliding down over the bird's tail.  The most difficult part is getting the second leg through the second loop without the first loop coming off the first leg.  (If you have an assistant, get them to hold on to something to prevent this from happening!)  I stick the blunt-end through the second loop to get it over the bird's second foot.  I then slowly work the second loop upwards on the second leg, always checking that the transmitter is sitting up high on the bird's back (you have to make sure it's not sitting on top of the bird's tail).  As with the first loop, you want to get the second loop as high as it can go on the bird's leg.  Ideally, the "drumsticks" of the bird's upper legs will keep the loops up high so that your transmitter remains firmly in place.
    When newly-tagged birds are released they are unsteady.  Of the five females that I radiotagged in 1999, thee flew away fairly easily, although they all wobbled on their first perches as they learned to compensate for the added weight.  Two other females had initial difficulty gaining altitude and landed on the ground after release.  Within five minutes of release, both of these females climbed trees by hopping up from branch to branch before making their first big flight.  All five females were tagged in the early- or mid-afternoon and all were behaving normally by evening.  Because the added weight of the transmitter is obviously a stress to the tagged bird, I minimized other manipulations to my subjects prior to attachment (i.e. I just weighed them, put the transmitter on, and let them go).  One must remember that even if a transmitter weighs only 5% of a bird's weight, it is still stressful to the bird and steps should be taken to minimize any ill effects.  Be prepared to follow newly tagged birds and re-capture them if they are not responding well to the transmitter or harness (I keep a butterfly net nearby when releasing tagged birds).
    The figure-eight leg harness is very easy to remove.  Slide a blunt end under the elastic (I usually go for the underneath, inside of the leg) and ease it down over the bird's "drumstick".  The whole thing will fall off before you can say "Please fall off, figure-eight leg harness."

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