Tuesday, January 18, 2011

Diary: The Bonelli's Eagle

With the new year I am introducing a new section to the blog called diary. Whenever this word appears in front of the post title it means I will be drawing on written texts from 19th and early 20th Century naturalists and comparing to the present. Photographs may not all be from now but will include some from my archive. Today, I start with the Bonelli's Eagle Aquila fasciata.

This impressive cliff-nesting eagle has attracted the attention of naturtalists for a long time. Sadly, this bird is very much in decline and is rarer each year. The European population is of the order of 1000 breeding pairs, 80% of which are in Spain with smaller numbers in Portugal, Greece, France, Italy, Croatia and Albania.

Its geographical range once included many coastal cliffs where it is now gone. A traditional site was the Rock of Gibraltar where it nested annually until last recorded in 1936 - the disturbance of the war years probably caused their disappearance. They nested below the old Signal Station (below).

Colonel Howard Irby in the second edition of his Ornithology of the Straits of Gibraltar (1895) tells us how " a pair nest annually at Gibraltar, at the 'back of the Rock', to the south of the Signal Station; there are never more than a pair, though there are four situations where there are nests, one of which has not been used for several years.

typical eyrie with two chicks

My friend and colleague Dr Juan Pleguezuelos of the University of Granada studied this habit of use of alternative nests and found that it was a way of avoiding parasite infestation. What is more he was able to show that birds which lined the nests with fresh branches of Maritime Pine Pinus pinaster had a greater breeding success rate. The chemicals produced by this pine acted as an effective insect repellent that kept the nests clean and the chicks healthy!

Bonelli's Eagles were once widely distributed across southern Iberia and our Pleistocene cave sites on the Rock have revealed evidence of their presence since at least 50 thousand years ago (above).

Irby tells us how "when not breeding they hunt together (male and female), one high above the other, suddenly stooping down on some luckless rabbit or else gliding off to take up a fresh aerial station whence to watch for their prey, which seems to be taken on the ground." The other favourite prey of this bird is the partridge.

Irby relates some fascinating accounts of situations and contexts unlikely to be found today. Ospreys also nested on Gibraltars cliffs and he tells us what happened when they came across each other: "on another occasion, in the same month, I saw a Bonelli's Eagle flying about not far from the Osprey's nest, when down swooped an Osprey, like a stone, striking the Eagle on the back and knocking out a lot of feathers. Shrieking out, they were bound together for a few seconds, and then separated, neither apparently the worse for the encounter, and each flying off towards their respective eyries.

Lt Colonel Willougby Verner, writing in 1909, was equally impressed by this powerful eagle. As a falconer and son of a falconer he "was intensely attracted by Bonelli's Eagle when I learnt that it was the same species which the Afghans employ for hawking small deer."

He further observed "as regards their structure, few Eagles, if indeed any, are so powerful for their size as is Bonelli's. Their massive legs and feet and abnormally large claws are seemingly out of all proportion to the rest of their body." 

It is a real pity that such a majestic animal should be suffering such a fate as we progressively eat into their territories and sever traditional connections that have maintained viable populations for tens of millennia...


  1. Very interesting thanks Clive. One of my favourite birds. Beautiful photos.