This is a diary about white birds. It is filled with rampant speculation.
Colorful birds catch our eyes: red cardinals, blue jays, orange orioles, purple finches. We have ideas about why these bright colors exist (mate choice and other social interactions) and the mechanisms that produce them (pigments derived from carotenoids in food, structural colors).
Brown and black birds are everywhere, their color derived from variants of the pigment melanin. Brown has obvious advantages for camouflage. Black can be a story for another day.
But what about white birds?
I’m fortunate enough to have a home with a view of a lake. I can always tell when the white pelicans are there.
As you can see, an enormous white bird is hardly inconspicuous. So why be white?
Many birds are white on the underside. In fact this is so common in animals that we don’t really think about it. This is called counter shading and it is particularly prominent in flying and swimming animals which can potentially be seen from above or below and makes them less noticeable when seen from underneath against a bright background. This pattern does occur in animals where the animals underside is not likely to be visible (snakes, mice) which implies there might be more to the story.
Many diving birds and other seabirds show countershading. Ones that swim a lot (penguins, loon, alcids) are dark above and white underneath. Ones that fly and dive from higher up tend to pale above and white underneath (gannets and boobies, terns, gulls). These colors allow them to be relatively inconspicuous to both predator and prey.
But what about birds that are all (or mostly) white? White animals attract our attention and there are large numbers of records of white (albino or leucitic) individuals in species that are not normally white.
But quite a few species of birds are normally white. There are birds that live in snowy environments (Snowy Owls, Ptarmigan, Snow Bunting) where the white serves as camouflage. There are a few white birds that live in rain forests (White Hawk, King Vulture, Bellbird). But the most obvious examples of white birds are in the wading birds.
Where I live we have three abundant wading birds that are either solid white (Snowy and Great Egrets) or mostly white with dark wingtips (White Ibis). The dark wing tips or primaries in many birds are thought to be an adaptation to reduce the rate at which these feather wear out. We also have closely related species that show counter-shading (Great Blue and Tricolored Herons) and species that are solid dark colors (Little Blue Heron, Reddish Egret, Glossy Ibis). The very distantly related Whooping Crane is also mostly white as is the Wood Stork. Just to complicate matters, juvenile Little Blue Herons are white and there are all white forms of both the Reddish Egret and the Great Blue Heron.
So what is it about wading birds that makes being white so common?
I can tell you that the white birds are very conspicuous and are noticed at much greater distances than their non-white kin. Which leads me to think that predation is probably not a major problem for adults. Maybe white rejects more light and keeps the birds cooler in sunny open places. Or maybe it just take less energy to produce white feathers compared to colored feathers.
An interesting line of speculation is that it is related to feeding mode. However ibises feed by probing in the mud and most of their prey can’t see them coming. Color should be irrelevant. Among the herons and egrets you have active open water feeders that are white (snowy egret), counter-shaded (tricolor), and solid (reddish egret). And although there are general trends in terms of habitat for these birds the five common species here (Little Blue, Tricolor, Great Blue, Snowy, and Great Egret) can be found just about anywhere.
Any thoughts?
I’ll finish with a mea culpa. I’m not going to be around this morning but will respond to comments in the early afternoon.