a.k.a. why is that Reddish Egret not red?

Colour morphs – another weird and fascinating feature of the bird world

I posted a photo of a Reddish Egret on Facebook tonight. The fact that it was entirely white seemed to need some explanation.

The bird is known as a white morph Reddish Egret. Like a number of other species, the Reddish Egret has two colour morphs: standard (which is slate blue and sort of reddish), and white. It is, to use the correct terminology, a polymorphic species.

So what exactly is a colour morph? Let’s start with what it isn’t. It is not a temporary condition. Unlike species that develop through phases as they mature (most gulls, for example), polymorphic birds do not grow out of one plumage and into another.

Nor is it an example of aberrant plumage (leucism, etc), which affects individual birds within a population.

In the case of polymorphism the differently coloured birds are members of the same species, are not even separated as subspecies. They are just colour variants, and like all polymorphic species, Reddish Egrets of the two morphs can freely interbreed and produce fertile young.

Why colour morphs?

There are various theories about why this variation occurs. One plausible idea is that polymorphism is a form of evolutionary insurance against unforeseen events. We can see how this would work when we look at the Peppered Moth.

This common nocturnal moth species, Biston betularia, is found across Asia and Europe, and also in North America ( where, confusingly, it is called the Pepper and Salt Geometer). It has two colour morphs: the standard light colored moth and a dark morph called the melanistic Peppered Moth. Historically the dark version was relatively uncommon, most likely because its dark plumage stood out better against the lichen covered trees on which they like to take their daytime rest.

Then came the Industrial Revolution. Heavy usage of coal to power the mills and heat homes changed the environment of England. Tree trunks turned black under the buildup of soot, and the light-coloured lichens were killed off. In a very short period of time the ratio between dark and light moths flipped. The first dark morph moth recorded in the Manchester area was in 1848. By 1895 dark morphs were up to 98%. And sure enough, once pollution controls were brought in and coal-burning declined the white morph reasserted itself. But the presence of a dark morph version helped ensure that the species survived.

Polymorphism in local birds

Polymorphic species are not that common: one estimate is that they make up about 3.5 percent of bird species, and only a few of those are passerines (“songbirds”). But within my home area of Eastern Ontario there are a number of examples:

• The Snow Goose has white and blue colour morphs. The blues are less numerous but not scarce, and they stand out well against a sea of the white birds. Mixed pairs (blue and white) are likely to produce a mix of blue and white offspring.
• Eastern Screech Owls come in two versions as well – rufous and grey. (It seems the grey is more predominant in our area but I don’t have an authoritative source for that).
• Most White-throated Sparrows have bright white supercilia (“eyebrows”) and a white central crown stripe . On the tan colour morph the crown is dark brown with a tan central stripe, and the supercilia are also tan. When they form pairs in breeding season, white-striped morphs tend to choose tan-striped partners, regardless of which gender is white and which is tan.
• The Dark-eyed Junco also has several distinct colour morphs, though in our area anything other than the Slate-coloured morph would be a rarity.
• And sadly, the facts are now in and the Redpoll, which at one time represented two species and a number of subspecies, has now been lumped into a single species with a variety of colour morphs. I don’t dispute the facts of the case as the genetic evidence is clear. But it did grieve me to lose a life list bird to an armchair de-tick.

  

Further afield

Looking outside the local area, there are two more colour morphs to be aware of in case they should put in an appearance. There is a white morph form of the Great White Heron, and Arctic Skuas (infelicitously called Parasitic Jaegers in North America) come in dark and pale versions. I have personally observed a breeding pair made up of pale and a dark morph skuas, as they made repeated strafing runs against two bipedal intruders on Westray in the Orkneys.

Not Colour Morphs

Not every species that shows variation counts as a colour morph. One might guess that “our” yellow-shafted Northern Flickers and the red-shafted versions in Western Canada are just colour morphs, but they are separated out at subspecies level – in fact there are eight extant subspecies, listed under the Yellow-shafted Group and the Red-shafted Group.

And despite claims by some, I don’t buy the inclusion of Feral Pigeons as polymorphic. Instead of seeing two or more recognizable colour morphs, we see wild variation in colours between individual birds, owing to the constant inbreeding between “wild” and domestic pigeons as the Feral species developed.
(BTW all of our city pigeons ultimately trace their ancestry back to a wild species called the Rock Dove, which is not polymorphic).

Amazing facts to dazzle your friends?

So there you have it. Probably more information about colour morphs than you wanted to know. But it does explain the white Reddish Egret!