Conspecific brood parasitism in the house sparrow

-We studied 94 clutches of the House Sparrow (Passer domesticus) in Newark, Delaware, to determine: (1) the frequency of apparent conspecific brood parasitism (CBP); (2) if the laying of eggs in a conspecific's nest is truly a parasitic interaction with the recipient; and (3) that indeterminate laying can predispose a species to CBP. Eight clutches yielded evidence of CBP; electrophoresis of egg albumin detected a genetically odd egg in 4 of 42 clutches, while 4 other clutches provided indirect evidence of CBP-visibly odd eggs in other than the ultimate position and two eggs laid in one day. An egg-removal experiment suggested that House Sparrows are indeterminate layers before the third egg is laid; thus a female can increase her fecundity by laying one or more parasitic eggs before completing her own clutch of normal size. A complementary egg-addition experiment showed that a recipient female may reduce her contribution to her own clutch by one egg if an egg is inserted before she lays her third one. Insertion after the third egg can result in an enlarged clutch to the potential detriment of both host and parasite. Received 19 May 1987, accepted 10 Sept. 1987. Females of over 50 species of birds are known to lay eggs in the nests of conspecifics (Yom-Tov 1980a, Andersson 1984). This behavior, variously termed intraspecific (or conspecific) nest (or brood) parasitism (e.g., Yom-Tov 1980a, Andersson 1984, Emlen and Wrege 1986), has been recognized since about 1900 (Weller 1959, Yom-Tov 1980a), but until recently it was treated largely as incidental, aberrant behavior. Its brief treatment under "dump nesting" and its exclusion from "brood parasitism" by Terres (1980) and Campbell and Lack (1985) are evidence of past perspectives. Our terminology throughout will refer to parasitism, even though the purist would object that we have not demonstrated cost and benefit of the behavior to the recipient and donor, respectively. Several recent authors have hypothesized which individuals should be parasitic and under what circumstances (Yom-Tov 1 980a, Andersson and Eriksson 1982, Andersson 1984, Emlen and Wrege 1986, Gibbons 1986). We summarize these as the: (1) "last resort hypothesis"-a female lays parasitically because, as a result of inexperience, social status, or accident, she lacks a nest of her own; (2) "enhancement hypothesis"-a female lays a clutch in her own nest and increases her fecundity by laying one or more eggs in other nests; (3) "pure parasite hypothesis"-a female lays all her ' Order of authorship determined by coin-toss. 2 Myrtle Bell Lane Lab., Dept. of Medicine, Duke Univ. Medical Center, Durham, North Carolina 27710. 3Dept. of Entomology and Applied Ecology, Univ. of Delaware, Newark, Delaware 19717-1303.