Song copying and cultural transmission in indigo buntings

Abstract

Abstract. First-year male indigo buntings, Passerina cyanea, copy the song of a neighbouring male during their first breeding season. To determine whether they assess their neighbours and copy selectively the songs of certain males, the following predictions were made from hypotheses of assessment and socially biased learning: (1) first-year males assess a neighbour and copy his song if he meets a minimum standard; (2) first-year males prospectively assess the breeding success and survival of several neighbours, then copy the song of one that has a high breeding success, and (3) first-year males sample their neighbourhood for a common song and conform to it. The 'song model' was identified and the male's breeding success determined in 580 cases of cultural transmission. More males than expected at random had either zero or more than two other males copy their song. One predictor of song transmission by potential 'models' was age (first-year or adult), a trait assessable by plumage colour. Adult males were more likely to transmit their song than were first-year males. In one population the bluer first-year males were more likely to be copied than were the browner first-year males. Also, a male that arrived early in the breeding season was more likely than a later arrival to be copied. The first-year males in a dense neighbourhood were more likely to copy a neighbour's song, but they did not generally conform to the most common song in their area. Although time of arrival and a minimal standard of plumage and behaviour may explain some variation between males in cultural success, much of the variation was unexplained. Many adults were not copied, because of the age structure of the population with more adults than first-year males. Also, many males that had a successful breeding season were not copied, perhaps because their success was unpredictable early in the season when most first-year males acquire a local song.