Swimmer's itch is caused by the accidental penetration of human skin by various species of avian schistosomes that naturally cycle in bird and snail hosts. Little is known about the ontogeny of avian schistosomes in their vertebrate hosts, especially in wild birds. Taking advantage of the abundance of common merganser (Mergus merganser) broods on northern Lower Peninsula lakes in Michigan, we obtained fecal samples from 97 common mergansers, focusing on ducklings (n = 75) of 13 different ages but also including birds that were 1 yr and older. Miracidia hatching from fecal samples were quantified per gram of feces to determine the timeline and reproductive output of naturally acquired schistosome infections. All ducklings 18 days or younger were negative. Beginning at 21 days old, some ducklings were passing a small number of eggs, with the percentage of ducklings passing eggs increasing with age. The number of eggs passed by ducklings remained low until approximately 7 wk of age. At 52 days and older, all ducklings were passing eggs and the number of miracidia produced was frequently many times higher, strongly consistent with published mitigation studies that duckling relocation severely reduces snail infections and case reports of swimmer's itch. Surprisingly, second-year common mergansers also passed high numbers of schistosome eggs but may contribute less to successful transmission to snails based on the published success of mitigation by duckling relocation. All brooding hens sampled were positive but passed low numbers of eggs. This is the first study of the development patterns of any avian schistosome in wild young-of-the-year birds, and the patterns are compared with the few known laboratory studies on worm development.