Egrets? I’ve had a few…

posted in: Tales of science | 16

 

Four of the five egret species found in Australia (the reef egret is missing). The first three are in breeding plumage.
Four of the five egret species found in Australia (the reef egret is missing). The first three are in breeding plumage.

Over the last couple of months four species of egret have been frequenting Dowse Lagoon. Sometimes I see them together in the same muddy corner near the bird-hide. They are the great egret Ardea alba, plumed or intermediate egret Ardea plumifera, little egret Ardea garzetta and the cattle egret Ardea ibis. And I’ve been wondering, how can four species of egret coexist like this? An underlying tenet of evolution is that species can’t coexist – at least for very long – if they are too alike. The best competitor is supposed to out-perform the others and fill the available niche¹. So what makes these four birds different enough that they are all here, peering beadily at me, flawless white with dagger-like beaks?

The most obvious difference between them is size. The cattle egret is about 50 cm high and barely reaches the belly of the great egret (about 1 m tall). And leg length influences where they feed, as egrets don’t like to get their feathery nether regions wet. So the larger egrets can hunt in deeper water, while the shorter ones stick to shallower water or moist (but not inundated) places like marshlands, grasslands and the shores of lakes. Great egrets have also been known to dive into deeper water to catch a fish, just for a lark.

I’ve noticed that the great egret doesn’t seem to do much. It stands around, looking down its beak at the water. Its main mode of hunting is to stand still and grab whatever comes within reach. It often wades into deeper water and some studies indicate that it prefers fish.

Extensive beds of dense plantlife have grown out of the shallow waters and swampy edges of the lagoon. It’s easy to spot the bright white plumed egrets foraging here. These birds walk slowly, sometimes peering over the tallish plants, and sneak up on their prey. They’ll take frogs, fish and insects, and whatever else looks tasty. Like the other egrets, the proportions of different foodstuffs in their diet vary from place to place, and probably reflect availability as well as preference.

Plumed egret hunting in emergent vegetation.
Plumed egret hunting in emergent vegetation.

The little egret doesn’t wade as deep as the great egret, but also has a preference for fish. It will actively run after fish, rather than standing and waiting. It also stirs the water with its feet, which have yellow soles that might scare the underwater critters. The other day I filmed one of these egrets doing some chasing and foot-stirring, although its yellow feet remained hidden. You can also hear a variety of birds calling in the background, including Torresian crows, a willy wagtail, figbirds and an Australasian darter.

The little egret also feeds with cormorants and pelicans and benefits when these birds drive small fish into shallow water.

I’m not sure why the cattle egrets are here, as there are no cows (but they also eat small cane toads, and there’s no shortage of them). Cattle egrets are found in groups, mostly in grassy and marshy areas, and hang around large animals or birds, snapping up the small animals that are disturbed. The alpha cattle egret gets the head of the cow, while those of lower rank have to make do with the rear.

So it would seem that there are enough differences in the sizes, feeding styles, habitats and preferred diets of these egrets to allow their coexistence.

But the cattle egret is a fairly recent immigrant – first known in northern Australia in 1948. Unlike other egrets, these birds are closely associated with cattle, and have benefited from the land use changes associated with cattle farming in Australia. But what happens during long and extensive droughts like the one now affecting much of Queensland? The pastoralists reduce the size of their herds, and the grasslands become mostly bare and dusty paddocks. Does this mean the cattle egrets go elsewhere for food, and end up competing with other egrets?

But that’s another story, for another day.

Feeding habits of four egret species.
Feeding habits of four egret species.

 

1. Defining a niche is actually much more complex than first thought, as species interact as fluctuating populations, and as different life-stages, in space and time. This article just scratches the surface of why these egrets can coexist by comparing their feeding behaviour. Species may also be prevented from performing at their best by other forces, so that competitive exclusion is delayed indefinitely. This can allow the coexistence of many similar species (Hubbell 2006).

References: Hubbell (2006) Neutral theory and the evolution of ecological equivalence; McKilligan (2005) Herons, egrets and bitterns: their biology and conservation in Australia; Morcombe (2000) Field guide to Australian birds; Pizzey & Knight (2013) The field guide to the birds of Australia (a later edition of Pizzey 1997); Recher et al. (1983) Foraging behavior of Australian herons.

Check out Why is the ibis often grubby and the egret always clean? and Is an aboriginal woomera like a heron’s neck? for more tales about egrets.

And also some lovely observations about egrets by Jane Way.

16 Responses

  1. What a fascinating post about the feeding habits of different egrets. Viewing your pencil sketches detailing the different sizes and appearances of four birds, together with the video of actual birds feeding in the lagoon, was very educational. It is so enjoyable to look at nature and biology from a scientific perspective, rather than from that of a writer and photographer. So much to observe, learn and enjoy in the great outdoors and we have it here in Queensland, in abundance.

    • Paula Peeters

      Hi Mary Thanks for reading and commenting. I think it’s fun to practice viewing nature from all sorts of perspectives: we can ask questions, analyze, document, draw, write, daydream, absorb or just be. Reminds me a bit of de Bono’s six thinking hats. Cheers, Paula

  2. I enjoyed these drawings and always get a kick out of watching little egrets do their foot waggling dance

  3. Hi Paula,
    Firstly, I loved the title! And the timing couldn’t have been better, too.
    My partner and I took a ride along one of Melbourne’s bike paths on the weekend (our first ride together since I moved here a few years ago) and I noticed a few Great Egrets (though I wasn’t 100% sure at the time) in one of the creeks flowing into the Yarra River. I was pretty stoked.
    I first saw Cattle Egrets where we were in Brisbane, but haven’t noticed any around Melbourne – I think we’re definitely too inner suburban here. The orange plumes during breeding season that the males get are quite fancy, you know.
    I really enjoyed reading your article and look forward to more.

  4. Paula Peeters

    Hey Dayna, thanks for reading and commenting, I’m glad you enjoyed it. I’ve always been a bit fuzzy on the differences of egrets until I wrote the post. Now that I’ve seen the foot-waggle of the little egret I don’t think I’ll be forgetting! Cheers, Paula

  5. Chow Chow (Tony)

    I (r)egret I have not read all the post yet Paula!!
    I am a pelican tragic so can you do a ‘thing’ on them.
    Loved your piece on the dirty Ibis. Ibis at Southport are immaculate. Don’t argue!
    How do I get my picture up like yours?
    See ya soon…T&E.xxxx

    • Paula Peeters

      Hi Mr Chow Chow, I’m glad you liked the ibis article. Can’t imagine why the Gold Coast ibises are so clean but I won’t argue :). Maybe those meter maids wash them down? If you want a picture to go with your name check out https://en.gravatar.com/ . Thanks for visiting and commenting! Cheers, Paula

  6. Thanks for finding my blog Paula. We share much in common in our love for nature and the desire for people to catalyse a better life through that passion.

    I really enjoyed this post. I often see egrets of all types around Currumbin on the Gold Coast. The elegance of the Great Egret as it stands still, waiting, waiting, is beautiful.

    Your illustrations are a lovely feature of your blog posts. I look forward to reading more…

    • Paula Peeters

      Hi Gail, Yes it’s good to make contact with you, thanks for reading and commenting on my blog. And for your kind feedback about my illustrations. I think your cycling endeavour is fantastic too – well done. I try to cycle as much as possible but I’m not keen on sharing the road with careless drivers. If we had more dedicated bikelanes separated from other traffic I’m sure there’d be lots more cyclists on the road. Cheers, Paula

  7. Thanks for this great post! I was also very hazy about egrets until this post. I love how your blog combines art and nature. I must catch up on reading your other posts. 🙂

    • Paula Peeters

      Thanks Jane – it’s good to swap notes with you on the fabulous nature of SEQ through this blogging thing!

  8. Wonderful post! I love, and admire, your collaboration of art and science in your posts. When trained as a field biologist in the early 1980’s, we were encouraged to include sketches in our field notes. Thirty-four years later and retired, I am trying to return to sketching and find it more difficult than learning a new language. 😉 Your posts are inspirational. Thank you!

    • Paula Peeters

      Hi Elzi thanks for your kind praise, I am thrilled that you find my posts inspirational. It’s great to hear you’re going back to sketching. Don’t be too hard on yourself – it’s never too late to learn. I am sure your sketches are probably better than you think, and the best bit is that they will also get better and better with practice. Cheers, Paula

  9. Robyn

    I loved this post on egrets, not to mention the clever heading! I found the information helpful, especially your whimsical sketches which cleared up some confusion I had on different species (or are they sub-species? Not sure.) of egrets.

  10. Paula Peeters

    Thanks Robyn for your egret-appreciation, and kind words about this post.