How healthy are juvenile eagle hybrids?

Study of feathers reveals nutritional status of hybrid and pure eagles.

“The available evidence contradicts the assumption that hybridization plays a major
evolutionary role [in animal evolution].” These are the words of biologist Ernst Mayr. At the time of writing (1963, in his book Animal Species and Evolution), he had a point.  Animal hybrids were rarely observed and often turned out to be sterile. These observations led to the idea that hybrids are generally less fit compared to “pure” species. But is this really the case? A recent study in the Journal of Raptor Research assessed this claim for two hybridizing eagle species.


A Greater Spotted Eagle (from:


Eastern Poland

Hybrids between Greater Spotted Eagle (Clanga clanga) and Lesser Spotted Eagle (C. pomarina) have been reported for over 150 years. For the past 30 years, researchers studied the breeding populations of these species in Eastern Poland. They discovered a substantial amount of mixed pairs. In fact, the proportion of broods producing hybrids increased by over 30% between 1996 and 2012. Hybrids seem to be doing quite well, but are they faring better than their “pure” counterparts?



To quantify the fitness of the birds, Gregorz Maciorowski and his colleagues relied on ptilochronology. This technique is based on the assumption that each growth bar on a feather represents a 24 hour period. The width of the growth bar is determined by amount of energy and nutrients invested during the growth process. So, the width of the growth bar indicates the nutritional status of an individual bird. The researchers also checked the feather for fault bars. These transparent bands are produced in times of stress.


The fault bars on the tail of a Sedge Warbler (from:


Water Regime

The feathers from nestlings of 54 Lesser Spotted Eagles, 42 Greater Spotted Eagles and 23 hybrids were compared.  There were no significant differences in the number of fault bars or the average width of growth bars. Hence, there seems to be no difference in nutritional status between the hybrid and the “pure” nestlings.

The researchers noticed a large number of fault bars, suggesting that the nestlings experienced stress. This could be due to the extreme environmental fluctuations in the Biebrza River valley. Variability in water levels could negatively affect the eagles because changing water levels may result in lower prey availability or less suitable foraging areas. How these changes will impact on the hybridization dynamics remains to be investigated.


A Lesser Spotted Eagle (from



Maciorowksi, G., Yosef, R., Väli, Ü & Tryjankowski, P. (2018) Nutritional Condition of Hybrid Nestlings Is Similar To That of Pure-Species Offspring of Spotted Eagles (Clanga clanga x C. pomarina).  Journal of Raptor Research 52(4):484-490.


This paper has been added to the Accipitriformes page.

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