of parental care, for example, is not known for many families of birds (Van Tyne and Berger 1971). Mating in birds assumes a variety of forms: monogamy, with partners showing attachment to each other, their territory, and their young (e.g., ring doves, Streptopelia risoria); polygamy, including polygyny, with males mating with two or more females (e.g., red-winged blackbird, Agelaius phoenicius), and polyandry, with females mating with two or more males (e.g., phalaropes, Phalaropidae); and promiscuity, with the sexes copulating without any other relationship to each other (e.g., ruff, Philomachus pugnax). It is not clear whether there are systematic endocrine correlates to these mating systems. Incubation stategies are as diverse as mating systems (Kendeigh 1952). Either, both, or neither parent may incubate. In the polyandrous spotted sandpiper (Actitis macularia), males perform most of the parental care. In contrast to the usual pattern of sex differences in hormone secretion, these males have higher estradiol levels than females, and nonincubating males have higher androgen levels than females or incubating males (Rissman and Wingfield 1983). These data suggest that androgen is not responsible for female aggression and that estradiol may be involved in parental care. In phalaropes, in which males also care alone for the young, incubation may be correlated with increases in prolactin (Hohn and Cheng 1965). These authors also report a correlation between incubation and pituitary weight (presumably reflecting prolactin secretion), and brood-patch formation in killdeer (Charadrius vociferous), in which both sexes incubate and develop brood patches, and red-winged blackbirds, in which only females incubate and develop brood patches. Conclusions about causal relationships must be made carefully, however, since Ball (1983) found that male and female barn swallows (Hirundo rustica) are equally capable of warming their eggs, even though males lack a brood patch. In Malee fowl (Leipoa ocellata), the males incubate, but not by applying body heat. They regulate the temperature of the nest by either opening the mound of rotting vegetation over the eggs or adding soil to the top (Frith 1956, 1967). To our knowledge, no research has been done on the endocrine correlates of this behavior. Various species reduce or eliminate the labor of parental care. Some birds lay their eggs in others' abandoned nests (owls) or expropriate others' nests and throw out resident eggs or young (e.g., the chestnut sparrow, Passer eminibey, Payne 1969). Cuckoo (Chrysococcyx spp.) fledglings are incubated and reared by foster parents, but are sometimes fed by their own (Friedmann 1968). Some birds never care for their young after laying their eggs in other species' nests. In the brown-headed cowbird (Molothrus ater), one of five parasitic cowbird species, exogenous prolactin does not induce incubation behavior (Selander 1960, Selander and Kuich 1963). In contrast to cowbird eggs, those of cuckoos closely resemble their hosts' in both size and color. Here each group is restricted to parasitizing the species it mimics. This continuum of behaviors presents interesting problems regarding hormonal regulation.