The ecology of song improvisation as illustrated by North American Sedge Wrens

Can the diverse styles of song development in songbirds be understood in an evolutionary context? Are song imitation and song improvisation strategies that evolved in identifiable ecological circumstances? Differences among C istothorus wrens suggested that song imitation was used in stable, resident populations by Marsh Wrens (Cistothorus palustris), but that song improvis ation evolved in the more nomadic populations of North American Sedge Wrens (C. platensis). Toward understanding this seemingly unique strategy of imp rovisation by North American Sedge Wrens, we reexamined song development in the laboratory and singing behavior and population movements among free-ra nging males. Nestling Sedge Wrens were collected in North Dakota and during their first year of life tutored with 10 Sedge Wren song types; throughout the experiment, males were in adjacent cages and could both hear and see e ach other. Songs of the laboratory birds were not close imitations of songs from the training tape or immediate neighbors; rather, songs were either i mprovised (different from but most likely derived from training songs) or i nvented (no similarity to other songs in their environment). In nature, mal es at a Nebraska site also had unique song repertoires, a pattern that is c onsistent with the improvisational mode of song development. Our field surv eys also verified that Sedge Wren populations are highly mobile, arriving a t or departing from breeding sites at seemingly odd times of the summer bre eding season. These data, together with evidence of song imitation among se dentary populations of Sedge Wrens in Central and South America, reinforce the idea that song improvisation among North American Sedge Wrens is a deve lopmental strategy. Because songs are improvised, each male is unique, but songs do not vary geographically; hence, it seems likely that males and fem ales can communicate with one another no matter where they find themselves in the geographic range of the species.