The great egret (Ardea alba) is a common migratory bird that can be found on every continent minus Antarctica. Chances are if you live nearly anywhere in the Americas below 45°N, you’ve seen this long-necked white water bird. These chances increase significantly if you live near a marsh, river, lake, or any other relatively shallow body of water where small fish dwell. I am lucky enough to have the San Jacinto River in my backyard – literally – and so get to see these large birds often.
I recently set aside a few minutes to observe one of these graceful birds that I often take for granted at Brazos Bend State Park. The object of my attention was wading quietly and alone in a small alcove of the lake, surrounded by grasses and small brush. No surprise there. After a few minutes in a staring contest (I won), the egret started to slowly wade farther into the cove, picking its foot up out of the water with every step in an exaggerated, almost cartoon fashion and making good use of those backward-bending knees (just kidding, they’re ankles). Once it appeared to find a more appropriate position, the egret began staring intently at the water in hopes of snagging a meal. Sure enough, after only about ten seconds, the egret slowly pulled back its head as if drawing a bow and let go – darting its head into the water with lightening speed. When it resurfaced, there was a little, slimy fish in its long yellow beak. I thought the egret would quickly swallow its lunch and be done with it, but it decided to play with its food instead. The egret dipped the fish in the water multiple times, as if teasing it, and then decided to let the fish go, only to snatch it up again seconds later. Perhaps the egret was trying to situate the fish for easier swallowing – so that it would slide down its long throat more easily. However, after dropping the fish into the water for the fifth time, the egret must have decided it wasn’t hungry and let the fish go for good. Whether or not the fish was still alive enough to swim another day, I’ll never know. It turns out this behavior is neither uncommon nor reserved solely for food; the great egret is a torturous murderer from the minute it pokes its little head out of the shell.
If one is even vaguely familiar with Biblical stories, one will know the basic premise of the story of Cain and Abel. If you don’t know the story, here’s what you need to know: Cain and Abel were brothers. Cain killed Abel. This was generally frowned upon. Incidentally, murdering one’s sibling, or siblicide, is still not widely accepted in most human populations. Yet in great egrets, this behavior is rampant and has been documented time and time again.
From a kin selection standpoint, killing your sibling may seem counterintuitive to increased fitness, and so has been studied by many researchers. The theory of inclusive fitness asserts that an individual organism can increase its own genetic fitness by helping or cooperating with its relatives. This is known as kin selection. Under this theory, it would not be ridiculous to assume that a great egret hatchling would want all of its siblings to survive, thereby indirectly increasing its own genetic fitness, assuming it didn’t hurt the fitness of oneself too much. However, this assumption would be false, as predicted by the theory when examining constraints on altruism and cooperation (Mock 1985).
Great egrets often lay three eggs during a given breeding season, with the first two eggs receiving the brunt of the testosterone (Pratt 1985). While egrets start incubating immediately after laying the first egg, they often don’t lay all three eggs at the same time. This is known as hatching asynchrony, and causes the three hatchlings to vary in size and strength (Mock 1985). The first two siblings are therefore larger, stronger, and more aggressive than their often younger, weaker brother or sister.
Due to competition for resources, it appears that sibling rivalry escalates to murder, as the stronger nestlings essentially bully and torture their less fit sibling to death. The reasoning goes as such: the weaker sibling is unlikely to prove fit anyway, so the two older siblings might as well get rid of it in order to gain more resources for themselves and make sure they reach their highest potential fitness. In this case, the benefits of increased resources outweighs the cost of killing a sibling.
You would think that the parents would step in and do something about this, but it might be that the parents secretly wish they had fewer mouths to feed so they could better survive to the next year. Mock (1987) found that parent egrets very rarely stopped sibling aggression (we’re talking 99.2% turning a blind eye). Let’s face it – it’s easier to fetch food for two hungry mouths than for three. This, paired with the probability that the third nestling will be the weakest link anyway, causes a lack of concern in the parents. Parkes (2005) notes that close proximity of nesting sites may increase the likelihood of infanticide by adult great egrets as well, because there is higher competition for resources and many of the parents cannot be sure that the nestlings are their own when the opportunity for adultery is so high. Life can be so cruel, especially if you are a baby great egret that ended up in the shallow end of the gene pool.
However, all hope is not lost for that unlucky third egg. In infrequent times of resource abundance, the third hatchling may well survive as the two older siblings see no threat to their food supply. But why is there ever a third egg? The third hatchling is essentially a “just in case;” evolutionarily speaking, you would do well to contribute as many offspring as you can in order to ensure that your genes survive, and this is much harder if every other egret successfully raises three hatchlings to your measly two during a resource-rich breeding season. Unfortunately for this hatchling, such times are few and far between.
Mock, D. W. 1985. Siblicidal brood reduction: the prey-size hypothesis. American Naturalist 125(3):327-343. doi:0003-0147/85/2503-0007$02.00
This paper introduces the reader to evolutionary causes of siblicide, namely ecological constraints such as food shortage, as well as hatching asynchrony.
Mock, D. W. 1987. Siblicide, parent-offspring conflict, and unequal parental investment by egrets and herons. Behav Ecol Sociobiol 20:247-256. doi:10.1007/BF00292177
This paper looks at the role that number of siblings plays in parent-offspring conflict, noting that two sibling nests have far less conflict between parent and offspring, while nests with a weak third sibling often see conflict between dominant nestlings and parents, essentially until the third chick dies.
Parkes, M. L. 2005. Inter-nest Infanticide in Ardeids. Waterbirds 28(2):256-257. doi:10.1675/1524-4695(2005)028[0256:IIIA]2.0.CO;2
This paper is an observational look at siblicide in a population of great egrets at Audubon Canyon Ranch (CA) by Michael Parkes.
Ploger, B. J., and Medeiros, M. J. 2004. Unequal food distribution among great egret Ardea alba nestlings: parental choice or sibling aggression? Journal of Avian Biology 35(5):399-404. doi:10.1111/j.0908-8857.2004.03253.x
This paper examines the possible reasons for unequal food distribution among great egret nestlings – sibling aggression or parental favoritism – by studying the results of an experiment in which some nestlings were allowed to fight and some were separated by Plexiglas and food distribution was recorded.
Pratt, H. M., and Winkler, D. W. 1985. Clutch Size, Timing of Laying, and Reproductive Success in a Colony of Great Blue Herons and Great Egrets. Auk 102(1): 49-63.
This paper looks at the average great egret clutch size during each breeding season over a 13 year period.
Great egret, Ardea alba, aiming for a fish dinner, taken by Mila Zinkova (public domain).
Great egret, Ardea alba, wading at Morro Strand State Beach (Morro Bay, CA), taken by Mike Baird (public domain).
The graph above from Pratt 1985 shows average clutch size and fledged size of great egret (Ardea alba) nests in Audubon Canyon Ranch (CA). It can be easily seen that while there are usually three eggs, only one to two nestlings survive.
Above is a graph of results from Ploger 2004. In graph A, the nestlings were not separated and were allowed to fight, while in graph B, the nestlings were separated by a Plexiglas barrier. Dominance rank of nestlings is indicated by alpha, beta, and omega, with alpha being the first born and omega being the last born. These results confirm that it is sibling aggression, not parental favoritism, that causes unequal food distribution among great egret (Ardea alba) nestlings.