EFFECTS OF RADIO TRANSMITTERS ON MIGRATING WOOD THRUSHES

We quantified the effects of radio transmitters on Wood Thrushes (Hylo cichla mustelina) using 4 yr of banding and telemetry data from Piedmo nt National Wildlife Refuge, Georgia. Flight performance models sugges t that the 1.6-g transmitter shortens the migratory range of wood thru shes by only 60 km, and the estimated migratory range is adequate to a ccomplish migration even with limited fat stores. We used two strength s of line, 5- and 9-kg test-strength braided Dacron, to attach the tra nsmitters using the thigh-harness method. We recaptured 13 returning r adio-marked wood thrushes, seven of which were still marked. Six of th e seven birds marked with the 5-kg rest harnesses lost their transmitt ers within 1 yr while all six of the 9-kg test harnesses were still at tached up to 21 mo later. Radio-marking did not reduce the return rate s of adults and immatures, and the transmitters did not cause radio-ma rked birds to lose more mass than banded-only birds. Wood Thrushes can successfully carry a transmitter during migration with no detectable negative effects. We recommend continued use of the thigh-harness meth od, but we encourage the use of 5-kg cotton line.