Avian siblicide, whereby some chicks are eliminated from a brood as a
direct consequence of sibling aggression, has often been interpreted a
s the outcome of a parent-offspring conflict over brood size. This con
flict, which arises from the fact that each offspring values itself mo
re than it values its siblings, results in offspring favoring smaller
families, of higher-quality individuals, than would be optimal for the
ir parents to produce. Considered as an evolutionary game, this confli
ct is complicated by two factors: before deciding to enforce their pre
ferred brood size by eliminating unwanted brood mates, offspring must
take into account the readjustments in parental investment that will n
ormally ensue from brood reduction. Likewise, optimal parental investm
ent will depend on offspring reaction to it. In particular, it is show
n that the cost that parents must pay to provide enough resources to p
revent siblicide will often be smaller than the benefit they obtain fr
om maintaining their preferred brood size. Siblicide can be viewed in
these instances as evolutionary blackmail, because its threat is suffi
cient to make parents increase brood provisioning levels. As a result
of the blackmail, sibling aggression may acquire a signaling role in t
he parent-offspring interplay.