It's easy to prove that big birds like gulls will cross oceans when they migrate, but tiny birds are another matter -- the trackers you need are usually enormous in comparison. The Vermont Center for Ecostudies wasn't daunted, though. It successfully tested extra-small geolocators on blackpoll warblers, migratory songbirds that are too small and light (0.4oz) to shoulder previous sensors. The newer device weighs just 0.02oz, and is no bigger than a dime. As you can see above, it was more of a minor inconvenience than a heavy burden.