The piercing distress screams given by winter birds are hypothesized to be cries for help (usually in the form of mobbing from altruists) and to have been evolved by kin selection. The screams are apparently directed at the screamer''s associates, not the predator, a point which supports the hypothesis of selection for altruistic behavior and weakens self-interest selection as a viable mechanism for their evolution. The screams are highly localizable, and attracted individuals are known both to mob predators and to experience risk in doing so; these facts argue that it is the attracted individuals which are altruistic, not the screamer. For 2 spp., the proportion of individuals that screamed when mist-netted diminished as winter progressed; this suggests that kin selection and not selection favoring reciprocal altruism resulted in the evolution of such behavior. The most powerful arguments that altruism, rather than self-interest selection, is the mode by which distress-screaming behavior was evolved are that permanent residents scream more than winter residents, and that among winter residents, diurnal migrants scream more than nocturnal migrants. The very important implication of this work is that measuring the frequencies of such screams in samples of mist-netted birds provides an ordinal index of kinship (or at least, stability of interindividual associations).