A geolocator is a small data logger (weight: about 0.5 g) including a light sensor and an internal clock. This clock functions like a timer which is started at the beginning of data logging. To record the intensity of the light in the surrounding of a selected swift, a geolocator is placed on its back before autumn migration starts. The swift is caught in the next breeding season and the geolocator can then be taken off for data reading. You receive data on the length of light and dark periods with the means of the recorded elapsed time. Thereby, you can calculate day length to determine the latitude of the swift’s location. To determine the longitude, you refer the time of midday to the Central European Time you started the data logging. This way, you can reconstruct migratory routes and find out wintering areas in Africa except for the time around equinox. Thus, you acquire useful knowledge of the swift’s life outside the breeding season.