Throughout the Netherlands several thousand Black-headed Gulls are ringed with two code-rings. The main question in the long-term colour ring research is: What is the survival and site fidelity of the Black-headed Gulls which winter in The Netherlands?
Each winter approximately 15 first winter birds and approx. 15 adult birds have been colourringed in a given city. Ringing is done as much as possible during the months November and December. In those months there is almost no migration of birds which travel through, and most Black-headed Gulls are at their wintering site. Moreover, in the last winter months there is still time available to read the bird rings.
Over the years the colours yellow, blue, red, white and black have been used. These rings have an inscription with two codes: letter-letter, digit-letter, letter-digit and digit-digits. Between the two codes is a dot (".") or a hyphen ("-"). The dot and the hyphen is not part of the code and you do not have to enter it. The letters B, D, G, I, O and Q are not used.