The 2015 January cover of Journal of Avian Biology shows a Ruby-throated hummingbird (Archilochus colubris) at an automated weighing station. The hummingbird is implanted with a small RFID tag and every time it visit the feeder its ID and mass is recorded.

You can read the full study here:

http://onlinelibrary.wiley.com/doi/10.1111/jav.00478/abstract

Photo by: Kenneth C. Welch Jr