Why is the ibis often grubby, and the egret always clean?

posted in: Tales of science, Writing | 9
An Australian white ibis looking grubby next to a great egret.
An Australian white ibis looking grubby next to a great egret.

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.

1. Handbook of Australian, New Zealand and Antarctic Birds: Ratites to Ducks Vol.1.

 

Australian white ibises seem to wash more than egrets, but still can't equal their whiteness.
Australian white ibises seem to wash more than egrets, but still can’t equal their whiteness.

 

9 Responses

  1. Melanie Venz

    “… 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.” An extension to this thought is Camila Ridoutt’s reseach (shown on ABC’s “Catalyst” in 2012, but not yet published?) which found that the uban ibis eggs were smaller in size and weight, and had 7-9 times higher concentrations of persistent organic pollutants (e.g. dioxins, PCBs, pesticides) compared to the eggs of inland ibis. Yet another example of an animal which, on the surface is advantaged, but is actually facing negative consequences of our unpleasant human traits and deeds. http://www.abc.net.au/catalyst/stories/3461541.htm

    • Paula Peeters

      Hi Mel, Thanks for the comment – the very first one on this blog – wooohoooo!
      Yes even though some animals seem to prefer living in cities, city life is not always good for them (just like it’s often not that good for people either!). Maybe the trick is to make the cities cleaner, greener, and safer for all of us.

  2. Belinda Daly

    Paula,
    I love the name ‘Paperbark Writer’ for your blog. Reminds me of a song penned in 1966, the year I was born….
    I am enjoying your blogs immensely, as I too am drawn to the remnant patch of paperbark swamp that is too wet for someone to build a new development on. Touching on your ideas about being inspired by nature, these places remain the only places left for us and especially future generations to come to appreciate what plant and animal life once covered this area. If we and our children don’t make connections to these now by feeling with our senses and being touched emotionally in our heart, we will neither know that these places matter nor care enough to ensure their survival into the future.
    Love your work and looking forward to more.

    • Paula Peeters

      Hi Belinda, thankyou for your kind words, I’m glad you’re enjoying the blog. Yes I also think we’re more likely to care for nature if we know it. But – as you allude to – there are many ways of knowing. We are limiting ourselves and our experience of the world if we only stick to learning the bare facts, and only absorb these second-hand, from screens or books.

  3. Linda Lee

    Hi Paula. Congratulations on getting your blog up and running. I’ve really enjoyed reading your snippetts on ibis, paperbark forests etc. I too like the clever name… I didn’t know you were a Beatles fan! Your paperbark article has inspired me to take a walk in the local wetlands this holidays (the same ones you talk about near your place) to reconnect. Lookig forward to reading more….

    • Paula Peeters

      Hi Linda thanks for the generous feedback. I’m really pleased I’ve inspired you to visit the wetlands, and look forward to hearing about what you see. Maybe you could do some drawing too? I find the music of the Beatles somehow warming and inspiring – it has a unique sort of energy I think. I’d been mulling over a name for the blog for a while – something involving paperbarks – and one day the name just popped into my head!
      Cheers
      Paula

  4. Hi Paula,
    This was a great article. Despite years of working in conservation I didn’t know either of these facts about birds preening. The great white egret is beginning to make an appearance in Britain along with spoonbills and the occasional ibis. It is exciting to see these birds but also shows just how much warmer it is becoming here. Warmer, but also, unfortunately, wetter.

    • Paula Peeters

      Hi Carol, thanks for your comment. Researching and writing this post was also a revelation for me. I’ve also discovered how few ecologists / birders / conservation workers knew anything about powder down (including me!) and its curious distribution among bird families, let alone the rumours of luminous herons. Just goes to show there are new discoveries about nature waiting to be made just under our noses sometimes. Even the simplest questions can have complex and fascinating answers.
      Interesting about the new bird immigrants to the UK, we are certainly living in a time of rapid change.
      Cheers, Paula

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