We examined the acoustic structure of the calls of 71 species of nestling passerines (both suboscines and oscines) and 4 species of nestling woodpeckers. Calls varied greatly in general acoustic structure, frequency (pitch) and duration. Some phylogenetic trends occurred, e.g. calls of woodpeckers and suboscines tended to be simpler than those of most oscines. Body size was significantly correlated with both maximum and minimum frequency. Age-related changes occurred in some species but not others. The results suggest some phylogenetic and developmental constraints on call structure, but do not rule out the possibility that selection acts directly on nestling calls. However, the hypothesis that cavity-nesters would have more easily localizable calls than open-nesters because of lowered predation risk was not supported by our study, which showed no significant differences in any frequency or temporal measures of acoustic structure between cavity-nesters and open-nesters.