Dutch Birding volume 26 (2004) no 1

Yellow-legged Gulls and hybrids breeding at IJmuiden

Since 1987, small numbers of Yellow-legged Gulls Larus michahellis have been breeding in mixed colonies of European Herring Gull L argentatus and Lesser Black-backed Gull L fuscus graellsii at IJmuiden, Noord-Holland, the Netherlands. Almost all were paired with Lesser Black-backed Gulls, producing hybrid young. By trapping and colour-ringing birds, it was possible to follow them over the years. Some of the adult hybrids returned to the colonies and started to breed, always paired with Lesser Black-backed Gulls. In 2002 the number of breeding Yellow-legged Gulls and hybrids was at least seven. On 12 May 1997, a female that had been breeding and producing hybrids since 1991 was trapped and a blood sample was taken for DNA-analysis. This indicated that she was not a pure Yellow-legged Gull, although she looked like one, but that one of her female ancestors had been a European Herring Gull. With this in mind, and with a growing number of known hybrids and possible back-crosses in the colonies, it has become increasingly difficult to be sure of identification when seeing a yellow-legged gull.

Fred Cottaar, Lutulistraat 42, 2037 CB Haarlem, Nederland