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.
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.
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.