EPOW - Ecology Picture of the Week

The Variable Little Shag

Little Shag (Microcarbo melanleucos brevirostris), Family Phalacrocoracidae
Hamurana Springs Nature Reserve, Rotorua, North Island, New Zealand

Explanation:  Here is a lesson in plumage variability.  We are in the spiritual Hamurana Springs Nature Reserve on the north shore of Lake Rotorua on North Island, New Zealand.  We previously explored a couple of interesting invertebrates here, but this week we put on feathers, as we encounter Little Shags of the cormorant family.  

What makes this species an interesting study is how variable it can be in plumages, ranging along a spectrum from all black, to black with a white throat, to pied with all white below.  Various sources cite this variation as consisting of two, or four, phases.  Mostly, the phases are described as: (1) a white-faced plumage phase, (2) a white-faced plus white-throated phase (shown above), (3) a white-faced and totally white underparts phase, and (4) a white-faced and smudgy underparts phase.  Some sources also cite an all-black phase, as well!

In the main photo above, and this one just below, is the white-faced plus white-throated phase, where the rest of the bird is in black.  

Here, the striking contrast of white throat in otherwise all-black plumage is more obvious: 

(And not to confuse the situation, fledglings can be all-black.  Or can have all white underparts but with a smudgy white face.  Hmm!)

And here is the white-faced and totally white underparts phase:

So ... why all the phases?  I've not seen any studies determining if some phases have greater cryptic value, or some other adaptive value.  Besides, the birds with the two very different plumage phases in my photos above were all from the same general location at Hamurana Springs Nature Reserve.  

Perhaps this is just a complex case of neutral selection, where variations have no specific, individual adaptive advantage or disadvantage, and just happen to arise through random genetic mixing.  

Regardless, Little Shags occupy forested and wooded riparian environments, as they feed largely on fish, freshwater crayfish, frogs, tadpoles, and some other aquatic species.  If their plumage variation somehow correlates with their specific tree-nesting and aquatic-feeding environments, at least I've not encountered any such studies.  Alert to research ecologists!