Intra- and Intersexual Functions of Singing by Male Blue Grosbeaks: the Role of Within-song Variation

—Songs of individual male Blue Grosbeaks (Passerina caerulea) typically begin with the same combination of elements, but the sequence and number of elements in the latter portion of songs vary. We examined the possible functions of within-song variation in Blue Grosbeaks at the Blue Grass Army Depot near Richmond, Kentucky, USA from 15 April to 31 July 2007. We examined singing rates and song characteristics of second-year (SY; n 5 6) and after-second-year (ASY; n 5 14) males, and conducted playback experiments (n 5 15) to identify the possible function of variation in song length. Male Blue Grosbeaks sang at highest rates prior to pairing, maintained relatively high singing rates during the post-pairing/pre-nesting and nest-building/egg-laying stages, and sang at lower rates during the incubation, nestling, and fledgling stages. These results suggest high singing rates are important in attracting mates and establishing territories, and lower singing rates may result from trade-offs associated with parental care. Males used longer songs during aggressive encounters with conspecifics and responded more aggressively to playback of longer songs. This suggests songs containing more elements signal increased aggression. Within-song variation may be an important way to vary song meaning for male Blue Grosbeaks, and perhaps other males in species with a single song type but repertoires of several different song elements. Received 13 February 2009. Accepted 17 June 2009. Singing by male songbirds serves a variety of functions, ranging from establishing territories and attracting mates (Smith 1991) to distracting predators (Ritchison 1991), communicating with young (Beecher 1990), coordinating nest exchanges with mates (Smith 1988), and informing females about threat of predation (Johnson and Kermott 1991). The way male songbirds use song to convey different messages varies among species. Most male songbirds have repertoires of several different songs (Catchpole and Slater 1995) and may use different song types in different contexts. For example, Dark-eyed Juncos (Junco hyemalis) have two acoustically and structurally different song types, ‘‘short-range’’ songs for communicating with females and ‘‘long-range’’ songs for communicating with other males (Titus 1998). Similarly, many wood warblers have repertoires that include two distinct types of songs that are used in different social and environmental contexts (Spector 1991, Byers 1995). Male songbirds in some species exhibit another level of song complexity by varying the way they sing, such as repeating or deleting certain song elements in subsequent renditions of a particular song type (Podos et al. 1992). Leitão et al. (2006) compared responses of both male and female Common Chaffinches (Fringilla coelebs) to songs of equal duration that varied in number of trill phrases; they found both males and females exhibited a stronger response to songs with more phrases in the trill. Males in many other species exhibit similar within-song variation, but little is known about the role this variation may have in communication with conspecifics. Each male Blue Grosbeak (Passerina caerulea) has a single song type that consists of a sequence of rapidly ascending and descending notes or elements. Songs usually begin with the same combination of elements with variation introduced by rearranging and adding elements in the second half of the song (Ingold 1993, Ballentine et al. 2003). Our objective was to examine the possible functions of this within-song variation using both observational and experimental approaches. Specifically, we examined song length (number of elements used), element repertoire, and song rate during different breeding stages and in different aggressive contexts. We also investigated potential differences in within-song variation between second-year (SY) and after-secondyear (ASY) male Blue Grosbeaks, as young often sing differently than older birds, especially during their first breeding season (White and Mooney 1999).