It is difficult to decide which assumption to test here. On one hand, in all animal marking studies we make the critical assumption that our markers do not affect behavior or survival of marked animals. On the other hand, the consensus opinion (also an assumption) that very small songbirds cannot be safely marked with today's geolocators was made quite clear to us when we proposed giving it a try. With an improved ultra-light-weight harness design and efficient deployment methods, we deployed geolocators on 40 adult male Golden-winged Warblers in 2013.
We published two papers in Condor: Ornithological Applications demonstrating our harness design and deployment methods and reporting that at least one species of 9-gram birds can carry geolocators through annual long-distance migration with no discernible deleterious effects. Therefore, our work has thus far supported the assumption of no effects of geolocators on small songbirds (at least using our methods).
Instructional materials are now available on the Methods page of this website and on the Minnesota Cooperative Fish and Wildlife Research Unit website. If you plan to mark small songbirds with geolocators in the near future, and you are concerned about using conventional, overly complicated, heavy harness designs and painstaking deployment methods, please give these methods a try. Also, please feel free to contact any of the authors with any questions.