I asked of my bird-friend Vojta Brlík about geocation of birds.
Vojta made two or three years ago research about bird ways.
GPS geolocators are expensive and weights a lot because it must has good batery with big capacity.
For studies of small birds he uses light level geolocation. It is better because it is cheap and not heavy. It is used in birds that return to the same place. Vojta studied ways of Acrocephalus arundinaceus. 

Principe:

Light level geolocator records time and intensity of light e.g. every 15 minutes.
Bird fly with geolocator in autumn to Africa and it returns in the spring. Ornithologists catch the birds and take off the device. Then they analyse data.

The time of sunrise and sunset are different in different places.

We  can calculate position from the data.

The pros and cons of light level geolocation
+ cheap, very simple princip, small weight
- less accurate, need of recapture

Vojta's results

Scientic article about light level geolocator you can read HERE.

Lukáš Bjolek