Egrets are beautiful, especially in their breeding plumage, when they sport long curved plumes and dramatically colored faces.
Those breeding plumes are so beautiful that demand for them—for decorating women’s hats—almost drove egrets to extinction, and concern for the heavily persecuted egrets is what gave rise to the bird conservation movement in the early 20th century.
Egrets earn those luscious plumes. Before they get to be adults in breeding plumage, egrets must survive a cutthroat childhood in considerably less impressive dress.
Egrets nest in colonies, often with several species together. The breeding colony I have been visiting is populated by Snowy Egrets, Great Egrets, and a few Black-crowned Night Herons. The nests are large, tangled messes of sticks high in trees that serve as a platform on which to incubate the eggs and raise the chicks. They’re not exactly the works of art that some birds make of their nests, but they seem to work for the egrets.
The nests are built very close together, but not because egrets enjoy each other’s company. The colony resounds with angry cries as neighbors land too close for each others’ comfort and must be chased away.
Why do they nest so close together, if they don’t like each other? Probably to reduce depredation of the nest. The more egrets in an area, the more watchful eyes on the lookout and the more sharp bills for defense. Too, local predators may get “swamped” by the sheer number of egret chicks in a colony, and simply be too full to eat very many of them.
The egrets may not like their neighbors, but they cooperate with their mates to build the nest and incubate the eggs. Sometimes when one parent comes to take over incubation duty from their mate, they present their mate with a stick.
So at least the birds are friendly within their families, yes?
The first few weeks of an egret chick’s life are its most dangerous. This is true for all birds, but not all birds need to worry about their siblings as much as they do about predators. Egret chicks may be killed by their nestmates, and the parents will not stop it.
In fact, the parents rig the game. Birds can only lay about one egg per day, but they can control when they begin incubating the eggs. Eggs don’t really start developing until incubation starts. This allows birds to control whether the eggs all hatch on the same day (synchronously), or on different days (asynchronously): to get synchronous hatching, wait to incubate until all eggs are laid. To get asynchronous hatching, start incubating before all eggs are laid, giving some eggs a head start.
Egrets have asynchronous hatching. This means that the oldest chick will be bigger and more developed than its second sibling, the second will likewise be bigger than the third, and so on. Bigger chicks are better able to compete for food, especially since egrets often don’t deliver food directly to a specific chick, but simply drop it in the nest and let the chicks fight over it.
If you are a third- or fourth-hatching chick, you are coming into the world with a huge disadvantage. You are physically smaller than your siblings and slower at eating. When siblicide occurs, and it often does, it is these smaller siblings that are killed.
Why do parents allow this brutality? Why not break up nestling fights, or incubate so that all of the chicks are the same size, or not lay that third or fourth egg at all?
The reason is that, cruel as it seems, this is an adaptive strategy. The parents do not know what kind of conditions they will have in any given breeding season. It might be a great season, teeming with easily-caught fish and low on predators, when even those shrimpy fourth-hatched chicks will survive. It might be a season high in predators, when eggs or chicks will be eaten, so that having a spare egg or chick will be necessary. One egg might even turn out to be infertile, again necessitating an extra egg for insurance. In all of these cases, a large clutch size is a good idea.
Or it might be a bad season with too little food, when only one or two chicks can be successfully reared. In that case, a large clutch size is a bad idea—but the parents can’t know this in advance. They have to prepare for both good and bad seasons. In the case of the bad season, they need a way to reduce the number of chicks with as little cost to their surviving chicks as possible.
This is where the hatching asynchrony and consequent size hierarchy among chicks comes in. If you have three chicks but only enough food for two, one needs to go. If the chicks are all the same size, they will all be relatively evenly matched for fighting, and all will have to fight very hard and probably take some injuries before one is killed. If the chicks are different sizes, however, the biggest chicks can kill the smallest without taking injuries themselves, leaving the parents with two healthy chicks.
An alternative would be to have the parents simply select and dispose of one chick. The problem with this is that it relies too much on the parent’s judgement, which might not always be right. The parents would have to have a good sense of the food requirements of the chicks, which constantly change as the chicks grow. The birds most likely to know whether the chicks are getting enough food are the chicks themselves: by leaving the siblicide to the chicks, the parents are leaving the decision to the experts.
When Mock & Ploger (1987) tested whether the egrets were pursuing the optimal strategy by moving chicks among nests to create nests with siblings all of the same age or with greater-than-usual age differences, they found that both versions were less successful than the natural system. Same-age chicks spent too much energy fighting all the time. Their parents brought them more food, but it didn’t result in any more surviving offspring than usual. Nests will exaggerated age differences also took more parental effort for no greater success, although it’s not entirely clear why.
Another experiment, by Ploger & Medeiros (2004), tested whether parents would distribute food evenly among their chicks if they had the chance. The scientists put clear plexiglass dividers into egret nests, separating the chicks so that they couldn’t injure each other directly or fight over food. (Not that the chicks didn’t try: they pecked the plexiglass.) While in regular nests, larger chicks get more food, in the experimentally divided nests it was the middle chick who was fed the most. This turns out to be hard to interpret, but it’s a neat experiment! And it does show that the parents do not fight the size hierarchy, as they don’t feed the smallest chick more.
We also know that the egrets’ strategy works because we’ve seen it. When the egrets were finally protected from being hunted for their pretty feathers, they were in real trouble, but they made a remarkable recovery. These considerations about how to raise the most chicks aren’t abstract: it’s those extra chicks in good years, and the efficient loss of extra chicks in bad years, that let these birds rebound from low population sizes.
Let’s take a moment to admire these chicks: fighting for their lives, and looking so strange.
Shall we compare those wings to an adult’s?
You’ve got a big of growing yet to do, baby.
It will be worth it: you’ll grow up into a handsome adult, stalking through the marsh and spearing fish on your bill, and these days nobody will try to turn you into a hat.
Quick note: lest you think that because I’ve been saying “egrets” while writing this post, herons are nicer birds—they are not. Herons and egrets are all in the group Ardeidae, and although not all of them have siblicidal family arrangments, there are both egrets and herons that do. Those Black-crowned Night Herons, for example, are just as bad as the egrets.
In fact, Medeiros et al. (2000) document a new type of sibling aggression that Black-crowned Night Herons seem to have invented: head swallowing. It’s… what it sounds like. Not cool, baby herons.
Medeiros MJ, et al. 2000. An unusual type of sibling aggression in Black-crowned Night Herons. The Condor 102(2):438-440.
Mock DW, Ploger BJ. 1987. Parental manipulation of optimal hatching asynchrony: an experimental study. Animal Behaviour 35:150-160.
Ploger BJ, Medeiros MJ. 2004. Unequal food distribution among Great Egret Ardea alba nestlings: parental choice or sibling aggression? Journal of Avian Biology 35(5):399-404.