Abstract: By feeding their youngs, adult birds increase the survival chances of their nestlings. But there are risks attendant upon the parents’feeding trips. How does a parental bird balance the pluses and minuses of its activities? The key to explain the diversity in parental behavior lies to the cost-benefit analysis. Even when parents invest only in their own progeny, they rarely distribute their care in a completely equitable fashion, even when the coefficient of relatedness between parents and all their genetic offspring is the same (0.5 as a result of sexually reproducing parents placing 50 percent of their genes on each egg or sperm). Sibling rivalry and siblicide help parents deliver their care only to offspring that have a good chance of eventually reproducing, while keeping their food delivery
costs to a minimum. It is conceivable that siblicide may have evolves only because of the fitness advantages enjoyed by offspring able to dispose of siblings that were competing for the same supply of food.

Key words: parental care, parental favoritism, siblicide, red mason bee, Nicrophorus vespilloide, egret, booby