Hybridization of the Mediterranean gull Ichthyeatus melanocephalus in Poland

Hybridization, the interbreeding of individuals from different taxa, is usually the result of contact between closely related species. It occurs when reproductive barriers between species are not fully developed during speciation. Gulls are a classic example of hybridization in birds. However, the group of so-called "small gulls" is poorly recognized in this context. Here, we summarised data on Mediterranean gulls Ichthyeatus melanocephalus hybridization documented in Poland for the 2006-2022 period. In the mid-20th century, this species began successfully colonising many European countries, and the first documented breeding pair of Mediterranean gull was recorded in Poland in 1981. The first documented cross-breeding between a male Mediterranean gull and a female black-headed gull Chroicocephalus ridibundus was recorded in Poland in 2006. Since then, a total of 25 interspecific pairs, between Mediterranean gulls and black-headed gull or common gull Larus canus, have been observed. Interspecific pairs with black-headed gull were the most numerous (76%). However, mixed pairs - formed by one of parental species and individual of the Mediterranean gull hybrid - were the most common pair category (N = 60). Neither pairs consisting of two hybrids individuals nor of Mediterranean gulls x common gull hybrids have been recorded in the colonies. The number of Mediterranean gulls fluctuated between 50 and 100 pairs during studied period. The number of interspecific breeding pairs remained constant over a 17-year period, one to three pairs per breeding season, while the number of mixed breeding pairs has increased over time and reached maximum 9 pairs per season. Overall, there is a clear increase in the proportion of mixed and interspecific broods in relation to the number of pairs consisting of two 'pure' Mediterranean gulls, such pairs approached 38% in 2019.