Here is a story that I enjoyed reading last night. The Black Swift is probably North America's least known regular breeding bird. It is a strange creature: they nest and roost in the spray around waterfalls, they fly out before day break and return after sunset, they forage high in the air at heights that make them impossible to see with the human eye from ground level. Most remarkably, their wintering grounds are unknown: between September and June of the next year, they are essentially lost from sight.

These are small birds and current animal tracking technology based on gps and radio signals is just too bulky to be used for them. A team from the Rocky Mountain Bird Observatory trapped four birds in summer 2009, and fitted them with a tiny backpack geolocator, nothing more than a light sensor, a clock and a memory chip, and presumably a miniature battery.

The light sensor was gauged such that every day sunrise and sunset moments were recorded, for a full year. The rest is history: sunrise tells you longitude and the time duration between sunrise and sunset (with the calendar date added in the equation) tells you latitude. It isn't awfully precise but it does not need to be for this purpose.

Out of the four birds, three were trapped the next year again; luckily Black Swifts are very faithful to their breeding sites. The locations for the three birds give a consistent picture, making it a fair claim that we now "know" the swift's wintering area.

Want to see the maps, read the fuller story? Check out the Earbirding site.

Beason, J.P., C. Gunn, K.M. Potter, R.A. Sparks, and J.W. Fox. 2012. The Northern Black Swift: Migration Path and Wintering Area Revealed. Wilson Journal of Ornithology 124:1-8.