A Tail-Mounted Radio Transmitter for Eastern Bluebirds

A Tail-Mounted Radio Transmitter For Eastern Bluebirds

T. David Pitts Biology Department University of Tennessee at Martin Martin, Tennessee 38238

The use of radio transmitters allows an investiga- served that while the majodtyof their birdsadjusted tor to follow individualanimals and study activities rapidlyto the radio transmitterpackage, six of the that would be extremelydifficult, if not impossible, birds experienced flight impairment. They also to describe otherwise. Cochran (1980) and observed a case of entanglement when an an- Kenward (1987) described many of the different tenna wedged in a crack at the entrance to the types of radio transmittersand attachment proce- nest cavity.The antennas on three other birdswere dures along with their advantages and disadvan- bent in a manner consistentwith previousentangle- tages. While radiotransmitters have been routinely ment. used on numerousspecies of birds, I am aware of only two reports, Sloan and Carlson (1980) and Caccamise and Hedin (1985) reported that small Allen and Sweeney (1989), that descdbe their use birdscould carry radio transmitterpackages weigh- on Eastern Bluebirds(Sialia sialis). ing up to 4.0 g (13.3% of Eastern Bluebirdbody weight) without significant problems. Howaver, In their study of the home ranges of Eastem Blue- Gessaman and Nagy (1988) detected adverse ef- birds,Sloan and Carlson (1980) used radiotrans- fects from radiotransmitter packages weighing as mitters that weighed 4 g (approximately 13% of littleas 2.5% of the bodyweight. While both of the the 30 g body weight of an Eastern Bluebird)and published studies involvingtelemetry of Eastern had 28 cm antennas. They attached each radio Bluebirds (Sloan and Carlson 1980; Allen and transmitterwith epoxy glue to the middleof a bird's Sweeney 1989) describedgenerally favorable re- back. Three of their six radio transmitters were func- suits,some birdsin each studydid experienceflight tional for three weeks while the other three radio impairmentor other problemsthat could be attrib- transmitters either realfunctioned or came off of uted to the radio transmitters. The weight of the the bird after 1-2 weeks. radio transmitters or the method and site of attach- ment may have caused the problems. Allen and Sweeney (1989) radio-trackedEastern Bluebirdsweadng radio transmitterpackages that I felt that a lighterweight radio transmitterand an weighed 2.4 g (8% of the body weight) including alternate site of attachment were needed in order the harness, which was constructed of to minimize the effects of the radio transmitter on monofilamentfishing line, and a 15 cm antenna. bluebird behavior. Also, ! needed a radio transmit- Like Sloan and Carlson(1980), they positionedthe ter package that could be installed by one person radio transmitter in the middle of the bird's back. since I usuallyworked alone. After reviewingthe They attached radio transmittersto 2 captive and literature (especially Cochran 1980, Perry et al. 28 free-ranging birds, 16 of which were observed 1981, and Kenward 1987) and discussingthe op- for 25 or more days. The batteries had sufficient tions with Robert R. Cohen (pers. corn.) who used power for approximately30 days of transmission. radiotransmitters on Tree Swallows(Tachycineta While Allen and Sweeney (1989) believedthat the bicolor),which weigh less than Eastern Bluebirds, weight of their radio transmitterpackage did not ! decided to use tail-mounted radio transmitters. adversely affect the birds, the weight was above In this paper I describe some of the advantages the 5% limit generally suggested for small birds and disadvantagesof such transmitters. (Cochran1980). Allen and Sweeney (1989) ob-

Page 106 North American Bird Bander Vol. 20 No. 3 METHODS RESULTS

I used radio transmitters that weighed 0.9-1.0 g, Duringthis study I placed radio transmitterson 13 includingthe 13.3 cm antenna. They were manu- EasternBluebirds (4 females, 9 males)and radio- factured by Holohil Systems Ltd. of Woodlawn, tracked each from 0-34 days. The initialreaction Ontado. Each radio transmitter was attached to of a bluebird when released after attachment of a an Eastern Bluebird on the ventral side of its tail at radio transmitterwas to fly to a nearby perch and the proximalend of the central rectdces(Figure begin preening. This is the same type of response 1). Four pairs of threads (size 1, surgicalcotton), that normallyoccurs after a bluebirdhas been cap- one pair from each corner of the radio transmitter, turedand releasedwithout a radiotransmitter (Pitts, were attached to the radio transmitterthrough tub- pets. obs.). After the bird preened, the radio trans- ing that had been incorporatedby the manufac- mitterwas usually hidden by the tail coverts and turer acrossthe antedflr and posteriorends of the the only visible evidence of the radio transmitter radio transmitter. I attached the radio transmitter was the antenna which protruded beyond the tip by wrappingand tyingeach pair of threadsto each of the tail by approximately8.2 cm. I did not see of two rectrices. I placed a drop of fingernailpol- any of the birdspeck or pull at their radiotransmit- ish on each of the eight knots. I attachedthe radio ters. All birds with radio transmittersappeared to transmitterswhile sittingin the seat of my truck fly normally,even on their initialflight after release. (with the windows closed) and holding the bird in a cloth bag on my lap. With the bird positioned Two birds lost their radio transmitters less than three ventral side up and with only the tail protruding days after attachment. Two other radio transmit- from the bag, I was able to attach a radio transmit- ters became loose but did not fall off of the bird. I ter in approximately 15 minutes. Most birds were suspect that in each of these cases I had failed to docile when their heads were covered with the dark completely cover one of more attachment knots bag, although some birdsstruggled and had to be with fingernail polish and the knots had subse- restrained by lightly wrapping a cord around the quently loosened. The other radio transmitters bag at approximately the mid-region of the bird. remained attached in their odginal position. One Each bird was released at the site of its capture bird carded a radio transmitter for 74 days before I and then observed with binocularsand a spotting was able to capture the bird and remove the radio scope. The birds were radio-tracked with a TRX- transmitter which was still firmly attached and not 10005 receiver(Wildlife Materials Inc., Carbondale, causing the bird any apparent harm. Illinois) and a hand-held three-element Yagi an- tenna. All of the observationsreported here were Beforereleasing a birdwith itsattached radio trans- made during 1987-1990 on my study area in mitter, I monitoredthe signal to vedfy that the ra- Weakley Co., Tennessee. dio transmitter was functioning normally. In spite of this precaution, I did not obtain any data from Figure 1. Ventralview of Eastern Bluebirdtail showingthe one bird because of a broken wire in the receiving positionof an attached radio transmitter(R), the transmitter antenna (A), and points(o) whree threads from the transmit- antenna. The radio transmitter was transmitting ter are tied to feather shafts. propedy when installed, but I discovered after re- leasing the bird that I could not detect the signal from distances greater than about 10 m. Unfortu- nately, the battery of this radio transmitter lost power before the problemwas identified.

I was able to monitor 5 of the 13 birds until the radio transmitter stopped functioning due to the loss of power from the battery. One of these radio transmitters(constructed in 1987) was predicted to function for about 15 days, while the others (manufacturedin 1988-1989) had a modifiedcir- Jul. Sept. North American Bird Bander Page 107 I I cult which used less energy and had an estimated patently due to the antenna having been caught, functionalperiod of about 30 days. I was able to probablyin barbed-wire,and then pulled free. While monitorthe 1987 model for 16 days and the oth- I did not investigate the effective ranges of radio ers for31, 31, 33, and 34 days, respectively. transmitters with bent antennas and make com- parisons with radio transmitters having straight Four birdswith radiotransmitters disappeared. Two antennas, it was my impressionthat receptionfrom of the radio transmitterswere functioning and two the radio transmitters with bent antennas was im- had stopped transmitting when the bird disap- peded. peared. i have no evidenceabout the fate of these birdsor their radiotransmitters. The birdsmay have DISCUSSION died on the study area and remained undetected or they may have moved off of the study area and The tail-mounted radio transmitters described here out of reception range. have some advantages over previouslydescribed radio transmitterpackages used on Eastern Blue- Three of the 13 Eastern Bluebirds on which I in- birds. The tail-mounted radio transmitters and at- stalled radio transmitters are known to have died tachment threads weighed approximately1.0 g in while the radio transmitters were still attached. in contrastto weights of 4.0 g (Sioan and Cadson each case I found feathers, includingthe rectrices 1980) and 2.4 g (Allenand Sweeney 1989) for ra- with the attached radio transmitter, scattered over dio transmitterpackages previouslyused on East- a small area. This evidence is consistent with the em Bluebirds. Birds receiving tail-mounted radio actions of avian predatorssuch as Accipiterspp., transmittersdid not require a period of adjustment Amedcan Kestrels (Falco sparverius), and East- before regaining normal flight capabilities.The use em Screech-Owls (Otus asio) which were present of epoxy glue to attach a radio transmitter to the on the study area. The remains of two of the dead skinor feathersof a birdis a provenmethod (Sykes birds were adjacent to barbed-wire fences. This et ai. 1990), but this method should not be used location may be relevant in view of my observa- duringcold monthswhen the birdmight suffer from tions of another bluebird with a radio transmitter. heat loss. To attach a tail-mounted radio transmit- While I was watching this bird and its mate near ter, removal of feathers is not necessary. Attach- their nest, the birdperched on a barbed-wirefence. ment of radio transmitters with a harness is also a When the bird attempted to fly, the antenna of the frequently used and reliable method, but the in- radio transmitterbecame wedged in a barb of the stallationand adjustment of the harness requires fence. The bird dangled upside down for about two personswhile the tail-mountedradio transmit- five seconds while it struggled to free itself, which ter can be attached by one person. it succeeded in doing as I watched. Later that day I recapturedthe birdand removedthe radiotrans- One of the disadvantages of using tail-mounted mitterwhich no longer possessed an antenna. radio transmitters is that the rectrices are molted duringlate summerand early fall. If telemetrystud- Under ideal conditions, such as a bird in flight ies are planned during the period of molt, the ra- above vegetation and hills, the radio transmitter dio transmittershould probably be attached by a signalscould be received from distances as great harness; tail-mounted radio transmitters would be as 1.2 km. if the bird were perched lower, which lostdudng the molt and attachment of a radiotrans- was the usual situation,reception was generally mitterto the skin might interferewith the growth of less than 0.8 kin. Since each of the birds usually new feathers. Another disadvantage of the tail- remainednear its capturesite, I normallyhad little mounted radio transmitter is that its positionis not difficultyin detectingtheir signals. I had no prob- at the center of gravity for the bird. By using a lem with signal "bounce"(i.e., reflection from tall lightweightradio transmitterand placing it at the hillsor mountains;see Mech 1983), probably be- base of the tail, this problem is minimized. I did not cause of the relativelyflat terrainon my studyarea. observe any problems with balance, flight, or ma- On several birds, the radio transmitter antenna neuvering in the birdswith tail-mounted radio trans- became bent, usuallyin a "J"shape. This was ap- mitters. However, heavier radio transmitters at- tached to the tail might hinder flight. Page 108 North American Bird Bender Vol. 20 No. 3 I The major disadvantage I perceive with tail- that they had been entangled.On the 32 ha cattle mounted radio transmitters is the problem of en- farm where my study was conducted, approxi- tanglingthe antenna in barbedwire. Tail-mounted mately4.0 km of barbedwire fence (withmore than radio transmitters are nearer to the posterior end 26,000 barbs)are present. These fencesare fre- of the bird than are back-mounted radio transmit- quently used as perches by bluebirds. Conse- ters. Consequently,when the tail-mountingposi- quently,the probabilityis highthat a bluebirdwith tion is used, a greater percentage of the antenna a radio transmitter would eventually have its an- protrudebeyond the tail of the bird. I knowthat in tenna cross a barb.of the wire and become one case a bird became temporarilytangled in a wedged. Bluebirdswithout radio transmittersare barbed-wire fence because the antenna was rarely harmed by barbed-wirefences. In their re- lodged;I suspectthat the bentantennas I observed view of avian mortalityon barbed-wirefences, Allen on several other birds were a result of the antenna and Ramirez (1990) did not find any records of being pulled through a barb. The antenna prob- Eastern Bluebirdsthat had been injured or killed ably became curved in the same way that flat rib- as a resultof collisionor entanglementin barbed- bon becomes curled when it is pulled across the wire fences. blade of scissorswhen making decorative bows for packages. I was able to duplicatethe shape of Attachment of the radio transmitter on the dorsal, bent antennas by placing an antenna over a rather than ventral, side of the tail might reduce barbed-wirefence and pullingthe antenna through the probability of entanglement. The use of a a barb. shorter antenna could also reduce entanglement butwould probably decrease the distanceat which I suspectthat entanglementin a barbed-wirefence the radio transmitter signal could be received. may have been a factor contributingto the death While my observationsindicate that tail-mounted of two of the three birds that died with radio trans- radio transmitterscan be used effectively on East- mitters attached. The two birds whose remains I em Bluebirds,I suggest that researchersconsider found beside barbed-wire fences may have had alternate methods of attachment when working in their radio transmitter antennas lodged in the areas where barbed-wire fences are common. barbed-wireand may have been unableto escape when a predatorapproached. Even if the bluebird ACKNOWLEDGMENTS were only momentarily restrained, the predator would have been more likely to capture the blue- Parts of this manuscriptwere preparedwhile I was bird. Allen and Sweeney (1989) noted one case on an Alma and Hal Reagan Faculty Development where the antenna of a bluebirdwith a backpack Leave from the Universityof Tennessee at Martin. radio became wedged in a crevice at the nest cav- The Universityof Tennessee at Martin Faculty Re- ity and they saw three other bluebirdswhose radio search Fund supported equipment purchases. I transmitters had bent antennas, apparently as a thank Robert R. Cohen for his numerous sugges- result of entanglement. Jackson et al. (1977) rec- tions.I am gratefulto Fred Anderkaof HolohilSys- ommended against the use of radio transmitters tems Ltd.for hisconstructive comments and prompt with antennas that protruded beyond the rectrices delivery of radio transmitters. of Red-cockadedWoodpeckers (Picoidesborea- lis) because the antenna frequentlycaught in the LITERATURE CITED bark of trees. Allen, D. H., and J. R. Sweeney. 1989. A transmit- I terminated my telemetry study because of the ter package for Eastern Bluebirds.Sialia possibilitythat birds with radio transmitters had 11:43-47. higherthan normalmortality rates. While onlythree Allen, G. T., and P. Ramirez. 1990. A review of bird of the 13 Eastern Bluebirds with radio transmitters deaths on barbed-wire fences. Wilson Bull. are knownto have died duringthe study,two other 102:553-558. bluebirds with radio transmittersmay have died, Caccamise, D. F., and R. S. Hedin. 1985. An aero- and the bent antennas of additional birds indicated dynamic basis for selectingtransmitter loads in birds. Wilson Bull. 97:306-318. Jul. - Sept. North American Bird Bander Page 109 II I Cochran,W. W. 1980. Wildlifetelemetry, pp. 507- Mech, L. D. 1983. Handbook of animal radio- 520, in S.S. Schemnitz, ed. Wildlife man- tracking. Universityof Minnesota Press, agement techniquesmanual. 4th ed. Wild- Minneapolis. life Society,Washington, D.C. Perry,M. C., G. H. Haas, and J. W. Carpenter.1981. Gessaman, J. A., and K. A. Nagy. 1988. Transmit- Radio transmittersfor Mouming Doves: a ter loadsaffect the flightspeed and metabo- comparisonof attachment techniques. J. lism of homing pigeons. Condor 90:662- Wildl. Manage. 45:524-527. 668. Sloan, N. E, and D. J. Cadson. 1980. Eastern Blue- Jackson, J. A., B. J. Schardien, and G. W. bird home range determination using ra- Robinson.1977. A problemassociated with dio telemetry. Inland Bird Banding News the use of radio transmitters on tree sur- 52:20-22. face foraging birds. Inland Bird Banding Sykes, P. W., Jr.,J. W. Carpenter,S. Holzman,and News 49:50-53. P. H. Geissler. 1990. Evaluation of three Kenward, R. E. 1987. Wildlife radiotagging. Aca- miniature radio transmitter attachment demic Press, London. methods for small passetines. •fldl. Soc. Bull. 18:41-48.

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