Partial correlations were used to analyse the relations between four morphological characteristics and length of prey for fourteen species of tern. The horizontal width of gape had the greatest effect in reducing the correlations of weight, lengths of bill and wing with length of prey to nonsignificant or negatively significant levels. The horizontal width of gape may be used to predict the optimal size of prey of a predator that eats its prey whole. Comparisons of the frequency-distributions of preferred widths of prey with those of actual prey eaten by various predators may provide information about interactions between potential competitors and availability of prey.