Our article about the mixed breeding pair Herring Gull x Lesser Black-backed Gull which was breeding in the IJmuiden Forteiland colony in 2017 was published in Dutch scientific magazine Limosa, volume 92.2.
Summary (source: limosa.nou.nu)
COTTAAR F, VAN KLEINWEE M & VERBEEK-COTTAAR J (2019) Mixed breeding pair Herring Gull Larus argentatus x Lesser Black-backed Gull Larus fuscus. LIMOSA 92 (2): 95-99.
In May 2017, a male Herring Gull was seen together with a female Lesser Black-backed Gull in the gull colony of IJmuiden, the Netherlands. The male had colour-rings green Y.CBC, the female was later colour-ringed with combination green Y.CHA. At the time of the first observation the pair was showing pair bonding behaviour: the male was feeding the female. During the breeding season the pair built a nest and incubated three eggs of which one egg hatched. Unfortunately, the chick only survived for a week. In the previous breeding season the male Herring Gull had been paired with a female (unringed) Herring Gull. Possible reasons to pair up with a female Lesser Black-backed Gull in 2017 include (1) voluntary switching to a new partner because of a previously unsuccessful breeding attempt, (2) forced switching because a previous partner does not return (deceased), (3) a partner is ‘claimed’ by a third individual, or (4) a shortage of partners of the same species (females of large gulls often breed in a colony different than the colony in which they are born). Although the reason for the peculiar mate choice remains unclear, a shortage of Herring Gulls in the colony could have made the male decide to switch to a Lesser Black-backed Gull. From the viewpoint of the female Lesser Black-backed Gull the decision to pair up with a male Herring Gull is intriguing because of the large number of Lesser Black-backed Gulls available in the colony. Documented observations of mixed pairs of Herring Gull x Lesser Black-backed Gull are rare. This was only the second case a mixed breeding pair was discovered in our well-studied gull colony.