Some song types of the Darwin's finch G. fortis persisted over 37 years with remarkable fidelity. An example appears in , just to the right of label A: one song is from 1961, and the other from 1999. Numerous other cases of close overlap are apparent (purple shading in ). More generally, the acoustic space of G. fortis songs changed little over the four decades, with no significant changes detected either in raw parameters or PCs, despite some trends bordering on statistical significance ().
While our data document persistence of some song types, we cannot assess with confidence whether novel song types emerged over the four-decade span. For illustrative purposes, we have labelled three song types of 1999 as B, C and D in . These song types are relatively isolated on the PCA diagram. While it is possible that these song types arose after 1961, it is also possible that these song types existed in 1961 but had just not been sampled. If these song types indeed emerged after 1961, it seems likely that they did not emerge de novo, but rather as modifications of the pre-existing song types. For example, song B is similar to song types from 1961 directly to the right of it on , except that it has more note repetitions. Likewise, song C is similar to 1961 song types directly below it on , except with more notes. Finally, song D looks similar to a cluster of 1961 and 1999 songs labelled E, but with the high and low-frequency components reversed in order. Hence, novelty in G. fortis
might emerge more from a recombination of elements, parallel to that described by Nelson et al. (2004)
than from a gradual modification of specific song types owing to inaccurate copying and improvization (e.g. Payne 1996
Mechanisms favouring the temporal stability of song types include preferences for specific (e.g. local) song types, or selection against songs with divergent acoustic features (songs that feature copy error or innovation or songs from a distant population, Wright et al. 2008
). In the case of G. fortis
, individuals appear not to have a choice of what song to copy; it is that of their social father. However, individuals may fail to copy their father's song with fidelity, and consequently suffer mating costs, to the extent that other individuals do not respond to their songs (Lachlan et al. 2004
). This hypothesis is consistent with data from Podos (2007)
, who found that male G. fortis
in the El Garrapatero population on Santa Cruz respond much more strongly to songs from their own population than to songs from the Bahia Academia population, 11 km distant. This discrimination occurred despite the fact that songs from the two populations overlap acoustically, an observation that suggests these birds respond most strongly to song types that are familiar.
Other mechanisms might favour novelty in songs of Darwin's finches. For example, Grant & Grant (1996)
showed that female G. fortis
tend to select males whose songs differ from their (the females') fathers. This tendency recalls the finding by Grant (1984)
that female G. conirostris
on Isla Genovesa avoid mates with songs similar to their fathers, perhaps a result of selection to avoid inbreeding. If females indeed avoid males singing paternal songs, males singing novel songs would presumably benefit. In order to balance familiarity and novelty, one strategy of G. fortis
males might be to recombine familiar song elements in novel ways. More data are needed to document the accuracy of copying and the prevalence of improvization/recombination in this species, and more widely across the genus Geospiza