Lifestyle choices or better beauty products?
The Australian white ibis often looks grubby, but the white plumage of egrets always looks freshly laundered – with a purity and glow that the makers of clothes detergents would die for. Both birds start out with white feathers. Both forage in muddy habitats, and eat foods that are slimy and sticky. So why does one look cleaner than the other?
The ibis is a common scavenger at parks, gardens and rubbish dumps and its grimy appearance is usually blamed on the dirty places where it feeds. But this doesn’t seem quite fair, because many ibises frequent natural wetlands, sports fields or open pastures, far away from rubbish bins and dumps, and often in the company of those gleaming-white egrets. Ibises and egrets also eat many of the same foods (fish, frogs, invertebrates) and both routinely poke their beaks into wet, muddy places. Their lifestyles are quite similar (although their feeding methods are not). Perhaps the scruffiness of the ibis has less to do with where and what it eats, and more to do with how well it can preen. For compared to egrets, ibises have only substandard preening tools, and they also missed out on the secret beauty product of the bird world.
All herons, including egrets, have two inbuilt combs: a pectinate claw on the middle toe of each foot, which they use for preening. While some species of ibis have a pectinate claw, it seems that the Australian white ibis has to make do with only a few indentations on the claw of its middle toe.¹ Egrets and ibises also differ in the shape and size of their beaks. An egret beak resembles a large, straight pair of forceps, while an ibis beak is longer and curved. While both beaks may be adequate for the coarser tasks of preening, I suspect the ibis beak might be less dexterous for the fine work of cleaning individual feathers. A study of wading birds (not including herons or ibises) found that birds with longer beaks spent more time preening. This may indicate that very long beaks – while useful for extracting critters from soft mud and sand – may not be very good tools for preening. But what of the secret bird beauty product, I hear you say?
Powder down is a curious substance exclusive to an elite and disparate bunch of birds – including herons, parrots, pigeons, bustards and tinamous. It’s a fine, keratin-based powder produced from special down feathers that are constantly growing. As these feathers – or pulviplumes – grow, their fine downy tips disintegrate to form powder. Birds use this powder when preening and it appears to be especially good at mopping up greasy or slimy substances. Talcum or baby powder is a good example of a similar kind of fine powder that can be used to remove greasy stains. Once familiar to only a select few bird taxonomists and taxidermists, similar kinds of ‘dry shampoo’ are now becoming popular for humans who prefer not to ‘wet shampoo’ their hair every day.
Powder down is the reason why the reportedly ‘glossy’ black palm cockatoo can appear to be slate-grey, and why cockatoo beaks and feet often appear greyish, and not shiny black. If a cockatoo’s beak becomes shiny black this may be a sign of beak and feather disease, a condition which affects feather growth. Since powder down feathers are constantly growing, and never moulted, the loss of these feathers is often the first sign of the disease.
The powder down feathers of herons are arranged in patches, and for a long time these were thought to be luminous and used by the bird to attract fish. Sadly, this myth has been dispelled, and perhaps just as well. Otherwise people might sell glowing herons-on-sticks instead of glowsticks at sideshows and festivals.
Poor ibises. If they look a bit grubby, this might be because they lack the best tools for the (preening) job, and not because of where they hang out or what they eat. They may look creepy with their long curved bills, and naked, black-skinned heads and necks. And their guttural honking and yawing noises don’t help. Some people are scared and revolted when ibises leap onto picnic tables and disembowel the contents of already overfilled rubbish bins. But it is not the fault of the ibis that its looks are unfashionable, its habitat has been reduced and replaced with suburbs and cities, and that it makes good use of human waste. Like many ‘nuisance’ animals, the ibis is probably just reflecting back on us some unpleasant human traits and deeds that we’d rather not think about.