Why do great egret chicks sometimes kill younger siblings?

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Serengeti National Park, Tanzania: herd of gnu (wildebeests) — Animal social behavior Herd of gnu (wildebeests) in the Serengeti National Park, Tanzania.

In the context of animal social behavior, great egret (Casmerodius albus) chicks engage in siblicide—where the first-hatched chick kills its younger sibling—primarily as a result of competition for limited resources.1

Because parental care, such as providing food, cannot always be shared among offspring without reducing the benefit to each individual, siblings often compete directly for access to these resources. In great egrets, the first-hatched chick typically kills its younger sibling to secure more food for itself. This behavior is a response to resource scarcity; younger siblings are only able to avoid this fate during years when food is particularly abundant.1