Data on nestling growth of brood parasites and their hosts are surprisingly few in the literature, Even the Brown-headed Cowbird (Molothrus ater), whose host relations have been studied in some other respects, has not been studied in any detail from this standpoint. This is particularly regrettable because the lack of host specialization and high incidence of multiple parasitism in this species recommend it for intensive studies of parasite-host growth relationships. Isolated or fragmentary records of growth in cowbirds are available in Friedman (The Cowbirds, C. C. Thomas, Springfield, Illinois, 1929), Pickwell (Trans, Acad. Sci. St. Louis 27:1-160, 1931), Herrick Wild Birds at Home, Appleton-Century, New York, New York, 1935), Nice (Trans. Linn. Soc. N.Y. 4, 1937; Wilson Bull. 51:233-239, 1939), Mayfield (The Kirtland's Warbler, Cranbrook Inst, Sci., Illinois, 1960), and Nolan (Ornithol, Monor, No. 26. 1978). Hann (Wilson Bull. 49:145-237. 1937) illustrated the growth of five cowbirds raised in three nests of the Ovenbird (Seiurus aurocapillus), Norris Wilson Bull, 59-83-103, 1947) provided data for five individuals raised by different host species, and Scott (Wilson Bull, 91:464-466, 1979) presented pooled growth data for nine individuals raised by three different host species. King (Auk 90:19-34, 1973) measured the growth of Shiny Cowbirds (Molothrus bonariensis) in nests of Rufous-collared Sparrows (Zonotrichia capensis) and found that broods of two cowbirds grew at a substantially slower rate than broods of one, He suggested that Z. capensis could rear a maximum of two cowbirds or four sparrows, or an equivalent combination.

In 1974 collected data on the growth relationships of Brown-headed Cowbirds and Dickcissels (Spiza americana) in prairie habitat in eastern Kansas (Konza Prairie Research Natural Area). The intensity of cowbird parasitism in this study was extremely high - I found nests containing as many as nine cowbird eggs and three host eggs, More than one cowbird was evidently laying in many of the nests, behavior that may present some intricate evolutionary problems with respect to clutch-size manipulation by cowbirds. Fifty-nine of 65 nests were parasitized 91%, and the mean number of cowbird eggs per parasitized nest was 3.1 (SD 1.74). Dickcissels raised up to five young in mixed broods of various composition. Brood composition at fledging in 27 successful nests (42% of the total) averaged 1.6 Dickcissels and 1.3 cowbirds.