Another difficulty in counting individuals may be their mobility and distribution in space. To estimate disperse populations, the so-called capture-recapture method is used.

To apply the capture-recapture model, it is fundamental to be able to mark the animals captured. Birds are marked using metal rings, as in the case of these European serins, Serinus serinus, ringed for a study begun by the Barcelona Natural Science Museum in the year 1990.
The method in fact contemplates four stages: capture, marking, recapture and calculation.