Cardinals produce an average of six eggs in each clutch. If there are more than six, the parents cannot adequately feed the young. If there are fewer than six, predators may destroy the entire clutch. This is an example of

Stabilizing selection

Explanation:

Stabilizing selection is a type of natural selection which favours the intermediate genotype and eliminates extreme genotypes (inferior and superior)Cardinals produce an average of six eggs in each clutch,here production of six eggs represents intermediate genotypeProduction of more than six eggs represents superior phenotype,this is not favored because parents in this case cannot adequately feed the youngIf there are fewer than six eggs then this represents inferior phenotype,where predators may destroy the entire clutch