EPOW - Ecology Picture of the Week

Steppe Eagle (Aquila nipalensis orientalis), Tawny Eagle (A. rapax rapax), 
and Wahlberg's Eagle (A. wahlbergi)
Masai Mara, Kenya

Explanation:  Are you seeing triple?  Africa has a remarkably rich diversity of birds of prey, including eagles.  But these three species, all of which I photographed in the same location in Kenya, in particular appear remarkably alike.

These are members of the eagle genus Aquila that form a set of sibling species.  Also called cryptic species, the term sibling species refers to species that very similar or nearly identical in appearance but that do not interbreed.  

Sibling species usually develop from some initial, single form.  However, in this case, these three species likely are not particularly closely related.  Their similarity in appearance might be more due to convergence of forms than a recent shared ancestry.

Tawny and Steppe Eagles, in particular, are very difficult to tell apart, and in fact were once thought to be the same species.  In the field, the main distinguishing feature is the length of the gape, that is, how far the yellow of the mouth extends under the eye.  In the photos to the right, note that the gape of Steppe Eagles extends to back of the eye, whereas those of the Tawny and Wahlberg's Eagles extend only to the middle of the eye.  Pretty subtle, eh?

Wahlberg's Eagles are a little easier to tell apart from their Aquila cousins here.  Wahlbergs have a slight crest of feathers on the back of the head (not always visible, though), tend to be a little smaller in size, and have longer and straighter tails (visible mostly in flight).  

All three species here can be highly variable in coloration and pattern, as well, and have dark brown as well as pale color morphs, to mix it up even further.  And there are several other Aquila species present in the same area, as well, although those are easier to tell apart from these species. 

One would expect potentially high competition among sibling species unless they have evolved different feeding behaviors and diets.  Steppe and Tawny Eagles both feed on carrion (dead animals) alongside vultures on the ground, and both chase insects at the edge of grassland fires; but Steppes tend to hunt a little more often by foot on the ground than do Tawnys and Wahlbergs which tend more to perch-and-pounce or soar-and-pounce on terrestrial prey.  

Sibling species complexes might be more common than usually thought, especially with inconspicuous life forms such as invertebrates, in part because they are so tough to tell apart ... but only to us.  In at least one case, sibling species of South American mountain tanagers apparently can distinguish themselves under ultraviolet light!