An evaluation of transmitter effects on the return rate of adult and juvenile Common Terns to a breeding colony
Advancements in technology have facilitated a rapid expansion in the range of species for which tracking studies can be implemented. Such efforts often require unique attachment methods or materials for which careful evaluation is required to fully understand potential tagging effects and subsequent implications. During the 2021 breeding season, our research team affixed two types of light-weight transmitters to adult and pre-fledgling juvenile Common Terns ( Sterna hirundo ) with leg-loop harnesses made of elastic cord. Our initial observations of these marked birds detected minimal differences in behavior and nesting success of marked adults, and failed to detect any notable impacts on the growth rate or behavior of marked juveniles, but were limited to a single year of data. To provide further insight into the potential effects of these tagging methods, we evaluated return and nesting rates for the 2022 through 2025 breeding seasons. We found that all individuals observed on the breeding grounds in the years following transmitter attachment had not retained the transmitter. The observed tag loss suggest that tag retention could be considered when planning future efforts, and limits our analyses to understanding lagged effects of the unknown duration of actual attachment. With this limitation in mind, we found that birds marked as adults with center-weighted and rear-weighted tags were 1.00 and 0.76 times as likely, respectively, to return to the colony site, and 1.09 and 0.77 times as likely, respectively, to be observed nesting when compared with unmarked controls. Similarly, birds marked as juveniles with rear-weighted transmitters were 0.54 as likely to return, and 0.72 times as likely to nest in subsequent years. These results suggest no negative impacts of center-weighted attachment on return or nesting rates, but moderate support declines in both metrics for birds marked with rear-weighted transmitters.