Considerations for radio-transmitter specifications on songbirds: color and antenna length matter too

Abstract

Researchers studying avian space use, dispersal, survival, and migration select tracking tags that optimize tag size (dimensions) and mass relative to the focal species to minimize risk to birds as they move and fly. Here we argue that antenna length and transmitter color are also critical tag design parameters that must be considered. Based on a review of the songbird tracking literature and unpublished cases, we found most cases of entanglement, which can result in bird injuries and death, occurred with longer antennas (>13 cm). However, such cases cannot infer rates of mortality or survival, nor may they be representative of entanglements across antenna lengths and studies using radio tags. Additionally, white or brightly colored transmitters that contrast with plumage, particularly of cryptically colored females and juveniles, may increase visibility to predators and result in higher mortality, or alternatively, cause a female to eject a nestling by mistaking the tag for a fecal sac or foreign object. Consequently, we call on researchers to report and publish cases of antenna entanglement, and for journal editors to encourage and applaud such reports, such that data can be compiled to inform guidelines on tag antenna lengths for different species. Future research is also needed to examine the impacts of tag color on parental behavior and survival of tagged birds. Meanwhile, researchers should strive to ensure that their tag color does not contrast markedly with avian plumages, while also taking care not to alter (i.e., do not color over) areas required for harness attachment, activating and deactivating tags. We hope such efforts will enable researchers and manufacturers to make more informed decisions about tracking tag design, particularly with respect to balancing the benefits of longer antennas, which provide greater power and detection distance, with risks of bird entanglement and mortality.

Keywords

• antenna length
• color
• entanglement
• mortality
• radio-telemetry
• survival
• tracking tags