Nice one Norm. This is a good example of sibling-rivalry: something that happens in humans and other animal species like these egrets. If you assume a monogamous mating system, sibs share 50% of their genes, but you share 100% of genes with yourself, so selfish behaviour like competing for and taking food away from your sib often pays off in evolutionary terms. There is a fine balance here though. Behaviours that might significantly reduce the survivorship of your sibling would be selected against because there would be a 50% chance that a copy of the gene or gene complex that codes for this would reside in the affected sib. In other words the gene would have a hard time surviving over generations because it frequently kills itself off.
Food resources vary between years and in lean years parents may not be able to feed all their chicks. If there is a hierarchy of age and size within the brood, the bigger chicks will tend to out-compete the smaller one for food and the small one will die. This "brood reduction" is a natural mechanism to regulate brood size to match the food resources available in a given year. To think of this another way, it may be better to produce one healthy chick rather than three sickly ones. In years of food abundance, there is enough food for all the siblings, even the smallest and youngest one.
While we were at the rookery, we met a lady from a rescue group that recovers prematurely downed or otherwise injured birds and then nurses them back to health for release. She allowed us to watch as she released four black-crowned night heron chicks. One is shown in the attached photo.
Interestingly, we were told that after great egret chicks fledge they continue to be fed by their parents for a while. But after the herons fledge, they are on their own.