Simply a warning note: sure some birds are hybrid, as myself has proven even before the quoted articles (see my paper in Dutch Birding) ...Indeed, I have followed the adult birds breeding, one a typical buteo and one a typical cirtensis, all the way till the juvenile fledged.
https://www.researchgate.net/public...zzard_on_Pantelleria_Italy_in_2008?ev=prf_pub
However, several photos around, even in the last Forsmann great book, should be proven as being hybrids or the so called "Gibraltar Buzzard", as they simply could be normal juv grey-brown cirtensis to my eye (or would need DNA to be proven). Indeed, cirtensis normally have a least (or even an unknown) grey-brown morph too often mystaken as hybrid or "Gibraltar Buzzard".
http://birdingfrontiers.com/2012/12/12/how-many-morphs-do-we-know-for-atlas-long-legged-buzzard/
Since very long time I have a paper in progress, about Atlas Long-legged Buzzard identification, variability and morphs, together also with one about the "hybrids and hybridisation" Crested/Oriental Honey Buzzard with Honey Buzzard that will report several new stuff and interesting material... Unfortunately, working to too many projects and papers (now also on Odonata and Carabus!) I am way behind. Silly behaviour (one should first finish something and then going ahead instead of caothically work with too many things altogether!)
The fact is that in museums, there are skins of SURE cirtensis, mostly juveniles, from southern breeding areas and/or from Tunisia, Lybia and Algeria (so NOT from Morocco only) collected in the '800 and early '900 exactly looking alike "Gibraltar Buzzard "... and that would prove extremely hard to be distinguished in the field from this kind of hybrids, leading to an over-exstimation of hybrids around North Africa and even Spain, Portugal or Sicily.
Also, during my numerous field trips to North Africa, I have seen cirtensis with this plumage (or if you like morph) in the southern areas of cirtensi breeding range ...therefore not only from North Morocco as the mentioned papers and handbook seems to imply.
Then, I think (or suppose) there is still much to discuss about...