An evaluation of transmitter effects on adult and juvenile Common Terns using leg-loop harness attachments

Marking birds with transmitters allows for the collection of data that are critical for fully understanding avian life history, but researchers must also be confident that performing such studies is as safe as possible for transmittered individuals. While much could be learned from tracking juveniles across dependency periods and first migration, doing so would require a harness-based attachment method which has not been evaluated on any species of juvenile tern. Therefore, we monitored the reproductive success and behavior of adult Common Terns (Sterna hirundo) and the growth and behavior of juvenile Common Terns after attaching transmitters to adults and juveniles with leg-loop harnesses made of elastic cord. We found that transmittered adults had similar reproductive success to untransmittered controls (hatching success for nests of transmittered adults = 0.553; nests of control adults = 0.665). Transmittered adults also expressed minimal behavioral differences from untransmittered controls when the groups were compared via paired treatment-control nest observations along with observations away from the nest. Transmittered juveniles had similar fledging success and growth rates to untransmittered control juveniles (fledging success for transmittered juveniles = 0.766; control juveniles = 0.817). Transmittered juveniles exhibited slight differences in behavior from controls, with increased rates of preening, although these differences did not appear to be detrimental. Finally, monitoring efforts during the breeding season following transmitter deployment found no difference in the return rate, nesting attempt rate, or hatching success rate based on treatment (P > 0.05). However, despite evidence of an individual retaining its transmitter into fall migration, no individuals retained their transmitters when resighted the following breeding season. While our results show that leg-loop harnesses made of elastic cord present a potential option for transmitter attachment to both adult and juvenile Common Terns, additional testing could provide further insight into potential long-term impacts.