EPOW - Ecology Picture of the Week

Two of the Same But Different

Pacific Reef-Heron (Egretta sacra), Family Ardeidae
Candidasa, Bali, Indonesia

Explanation:  Ah, the wonderful tropical beaches of Bali, Indonesia!  Here, along the southeast coast, I arose early to enjoy the low tide and ... what am I seeing here?

These are clearly herons.  Or egrets.  But, who?

After some exploration and investigation, I determined that these very different-appearing birds of the rocky shoals are ... the same.  

Well, the same species.  Pacific Reef-Herons.  Also known as Eastern Reef-Egrets.  

They are two color morphs of the same species, that appeared individually on successive days at this same location.  This species is strictly coastal, usually solitary, and likes to fish at low tide.

The dark phase in the left photo, above, shows its characteristic neck plumes.  That one was easy to identify.

It was the light phase, in the right photo, that first threw me.  After some research, I realized that it was not an Eastern Cattle Egret or Little Egret (obviously way too big), or a Great Egret or Intermediate Egret or Chinese Egret (legs were greenish, not black), all of which also occur in white plumage.  This light phase Pacific Reef-Heron (Eastern Reef-Egret) has a dull yellow bill with a dark tip, and pale greenish legs, just as in my photo, differentiating it from all other contenders.  

Puzzle solved!

Now, why both color morphs evolved, and occur on the same rocky shoals, is perplexing.  It is a case of sympatric balanced polymorphism, where different phenotypic forms occur in the same place.  

I do have to wonder if there is any "hybridization" between the two forms, creating some intermediate form, or if one form always genetically dominates.  And if there is some competition between the two forms which is why they appeared separately.

Calling all research ecologists!