Are Black Kite hybrids moving into Europe?

Photographs reveal more birds with features from an eastern subspecies.

Did you know there are three subspecies of Black Kite (Milvus migrans) across Eurasia? The western subspecies (migrans) can be found from Europe into Russia, where it is replaced by the eastern subspecies (lineatus, also known as the Black-eared Kite). The third subspecies (govinda) occurs in India and Southeast Asia. All three subspecies overlap in distribution and might interbreed in these contact zones. In his book on European raptors, Dick Forsman stated that Black Kites with characteristics of the eastern subspecies (lineatus) were increasing in Europe. These birds might represent hybrids from the contact zone between the western and eastern subspecies. A recent study in the Journal of Ornithology tested the claim by Dick Forsman by analyzing the pictures of Black Kites in Europe. The researchers used a set of morphological features to discriminate between the different subspecies and potential hybrids.

Distribution of different Black Kite subspecies. From: Andreyenkova et al. (2019).

Photographs

The careful analyses of numerous pictures revealed observations of 65 Black Kites with lineatus-features in Europe. The sightings of these peculiar birds increased over time, with a notable rise in 2018 and 2019. An interesting result that raises many questions. First, does this pattern really represent more lineatus-like birds moving into Europe? Or can this result be explained by a significant increase in the number of birdwatchers and photographers (with better equipment) in Europe? More systematic monitoring of Black Kites – perhaps even using GPS-trackers – might be needed to better understand the movements of these birds.

Second, are these lineatus-like birds really hybrids? The researchers mention that carotenoid supplementation in the food can enhance the yellow color in the beak and legs of the birds (an important feature in identifying these hybrids). Intermediate coloration of these traits can thus be explained by both hybridization and diet. Again, more research is warranted here. In addition, there is a lot of overlap in morphological characters between these subspecies. It is thus possible that there is clinal variation across the range of the Black Kites. Taxonomists have classified the extremes of this cline as distinct subspecies (migrans and lineatus), but there might be a whole range of intermediate phenotypes. Indeed, a recent opinion piece in the journal Ibis highlighted the difficulties of clinal variation in taxonomic decisions. We need more insights into the morphological and genetic variation across the entire range of the Black Kite before we can confidently assess whether the lineatus-like birds in Europe represent hybrids or not. An exciting question to explore.

An increasing number of Black Kites with lineatus features have been observed in Europe. From: Skyrpan et al. (2021) Journal of Ornithology.

References

Skyrpan, M., Panter, C., Nachtigall, W., Riols, R., Systad, G., Škrábal, J., & Literák, I. (2021). Kites Milvus migrans lineatus (Milvus migrans migrans/lineatus) are spreading west across Europe. Journal of Ornithology162(2), 317-323.

Featured image: Black Kite (Milvus migrans) © Вых Пыхманн | Wikimedia Commons

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