Dark and light (fulvescens) morphs of Greater spotted eagle at Al Jahra Pools, Kuwait, January 2022. © M. Gholoum
Quite a few raptor species display different plumage colours. For some, like the Common buzzard (Buteo buteo) the variation seems to be continuous from very light coloured birds to very dark and all the variations in between (Look here). In other species, the variations segregate into two rather distinct types: dark and light. These different colour types are called "morphs", and some well-known examples are found in Booted eagle (Hieraaetus pennatus) and Eastern screech owl (Asio otus). See below. These colour differences sometimes earn the individuals a sub-specific classification, or at least a special name. So, the grey variation of the Screech owl is sometimes called the McCall's morph or Asio otus mccalli, and the red variation is sometimes called the Northern morph. In the past, colour variation sometimes lead taxonomists to believe the variants to be completely different species.
Two colour morphs are also observed for Greater spotted eagle (photo above). By far the most common is the dark morph, in which the feathers are dark brown. Much rarer is the light or fulvescens morph. Little is known about this morph. except that it is rare, and it seems to occur more commonly in the eastern part of the breeding distribution, and is more common in winter in Arabia and India. An informative article on this can be found here http://docs.sibecocenter.ru/programs/raptors/RC28/RC28_89-104_ShortReports.pdf, starting on page 96. The lead author on that article tells me that there have been more observations of fulvescens in that part of Russia in recent years. At least two light coloured morphs of Greater spotted eagle have been wintering at Al Jahra Pools during 2021-22.
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