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

Intermediate color morphs of the Common Buzzard are most successful

Why do intermediate morphs do better than light and dark ones?

When I am driving on the Belgian and Dutch highways, I always keep an eye on the roadside. Occasionally, a Common Buzzard (Buteo buteo) will be sitting on a pole, staring into the distance. The plumage patterns on the chests of these Buzzards vary from almost completely white to dark with a striking crescent of brighter feathers. The occurrence of such color morphs has fascinated birdwatchers and ornithologists for decades.

A 2001 study of the German Buzzard population found that intermediate morphs are more successful – in terms of survival and reproduction – compared to the lighter and darker morphs. To explain this finding, the researchers turned to the putative genetic basis of this trait. Let’s assume that color morph is determined by a single position (i.e. a locus) in the genome with two variants (i.e. alleles): L for light and D for dark. A bird with two L-alleles develops into a white morph, while a bird with two D-alleles will become a dark morph. The combination of L and D, however, results in an intermediate morph. In some traits, a heterozygous combination (LD) has a higher fitness than a homozygous combination (LL or DD). This phenomenon is known as heterozygote advantage and the researchers suggested that this is driving the frequency of intermediate Buzzard morphs in Germany. However, recent studies in the Netherlands question this explanation.

A dark Buzzard morph © Lukasz Lukasik | Wikimedia Commons

 

Intermediate Nestlings

First, the genetic basis of the Buzzard morphs. The German study assumed that this trait is encoded by a single locus with two alleles. This prediction can be tested easily using the Mendelian genetics that you probably learned in high school. If two intermediate morphs pair up, you can predict the likely morphology of their offspring: 25% chance for light (LL), 25% for dark (DD) and 50% for intermediate (LD and DL). To clarify this calculation, I added a Punnett-square below. Elena Frederika Kappers and her colleagues tested this prediction using data from more than 200 Buzzard families. In contrast to the expected 50% intermediate offspring, the researchers found much more intermediate nestlings (74%), indicating that this trait is not encoded by a single gene but probably under the control of multiple genes (i.e. the trait is polygenic). From population genetic theory, we know that heterzygote advantage is not always an effective mechanism to maintain variation in polygenic traits.

A Punnet-square showing the different combinations of L and D alleles. The while boxes indicate the alleles from the parents. The colored boxed show the morphs colors (light, intermediate and dark) depending on the combination of alleles.

 

Parasites?

The genetic underpinnings of Buzzard plumage might make heterozygote advantage less likely, but we cannot discard it completely. In fact, a recent study in the Journal of Evolutionary Biology showed that, similar to the German population, Dutch intermediate morphs performed better than their light and dark cousins. It is still unclear what ecological factor determines the success of intermediate morphs. The researchers speculate that intermediate morphs might breed in the best territories and have a competitive advantage. Or perhaps intermediate morphs are less susceptible for parasite infections (which could actually be due to heterozygote advantage). Indeed, another study showed that Buzzard chicks with darker plumage were more susceptible to infection by carnid flies (Carnus haemapterus) while nestlings with lighter plumage had a higher infection rate with the blood parasite Leucocytozoon toddi.

Light and dark morphs might be more susceptible to parasite infections. Dark morphs have more Carnus-infections (left) and light morphs have more blood parasites. From: Chakarov et al. (2008) Functional Ecology.

 

More Intermediate Morphs

Regardless of the underlying mechanism, it seems that intermediate Buzzard morphs will become more common in the future. Long-term data from the Dutch population showed that the frequency of intermediate morphs increased steadily. This patterns is likely due to the combination of assortative mating (pairing up with a partner that looks like you) and the reproductive success of intermediate morphs. Breeding pairs consisting of intermediate Buzzards produced more offspring and these young birds mostly had intermediate plumage themselves (ca. 75% of the nestlings belonged to the intermediate morph). It does not take a mathematical genuis to see that this positive feedback loop will lead to more intermediate morphs in the coming generations. I will keep an eye on it while I am driving on the highway.

 

References

Chakarov, N., Boerner, M., & Krüger, O. (2008). Fitness in common buzzards at the cross‐point of opposite melanin–parasite interactions. Functional Ecology22(6), 1062-1069.

Kappers, E. F., de Vries, C., Alberda, A., Forstmeier, W., Both, C., & Kempenaers, B. (2018). Inheritance patterns of plumage coloration in common buzzards Buteo buteo do not support a one-locus two-allele model. Biology letters, 14(4), 20180007.

Kappers, E. F., de Vries, C., Alberda, A., Kuhn, S., Valcu, M., Kempenaers, B., & Both, C. (2020). Morph‐dependent fitness and directional change of morph frequencies over time in a Dutch population of Common buzzards Buteo buteo. Journal of Evolutionary Biology33(9), 1306-1315.

Krüger, O., Lindström, J., & Amos, W. (2001). Maladaptive mate choice maintained by heterozygote advantage. Evolution55(6), 1207-1214.

Featured image © Ronald Huijssen | Wikimedia Commons