EPOW - Ecology Picture of the Week

The Ghost Eagle of Africa

Tawny Eagle (Aquila rapax) color morphs
Masai Mara, Kenya, Africa

Explanation:  What is this ghostly white eagle, and how is it related to this all-brown eagle?  

These two photos are of the same species of eagle ... in the same location in southern Kenya, east Africa.  How can they appear so different, yet be the same species?

These are Tawny Eagles of Africa, a locally common and widespread species.  The "ghost" form on the left is actually a "pale form" individual, often appearing in juvenile birds.  Tawny eagles have highly variable plumage coloration, but most appear, well, "tawny" brown.  It is the occasional pale form that really stands out.

 
Two more views ... of the "ghost eagle" pale form on the left, 
and the normal brown morph on the right.

Interestingly, the closely-related Steppe Eagle, also found in many of the same locations and habitats as the Tawny Eagle, can also have a pale juvenile form that looks nearly identical.  Telling these two species apart entails looking very closely as some quite subtle characteristics; we explored these "sibling species" in an earlier EPOW episode.  

So this "pale form" -- also referred to in the bird books as a "blonde morph" -- of the Tawny Eagle overlaps in range with the regular brown form.  But why does it occur?  Is there some adaptive advantage?  Does this color convey some special reward?  Tawny Eagles, like many other species of eagle, feed on carrion (dead animals), and are known to follow grass fires that scare up insect and small bird prey items, and sometimes steals prey from other predators.  How could a "blonde" color aid in this?

One explanation -- perhaps the most parsimonious (simplest) and one that should be accepted  until shown otherwise -- is that this is an example of neutral selection, that is, there is neither advantage nor disadvantage conferred by the various plumage forms.  Further, color morphs are fairly common among other birds of prey (and other species), suggesting that something in the inherent genetic makeup allows for such variation throughout many species.  

That said, one study of color morphs of the Common Buzzard (Buteo buteo) did find that its 3 color morphs seemed to vary in overall lifetime reproductive success, although it was not understood how this happens.  The study found that light-colored males were more aggressive toward other predators, although it was the dark-colored females that showed this aggressive tendency.  And that Common Buzzards defended their territory more aggressively against other Common Buzzards of the same morph type than against other morph types.  The study concluded that the genetic basis of the various color morphs indeed as "far-reaching behavioral consequences."  

As for Tawny Eagles, such studies may await to be done.
  

Information:
     Boerner, M., and O. Krüger.  2009.  Aggression and fitness differences between plumage morphs in the common buzzard (Buteo buteo).  Behavioral Ecology 20(1):180-185.