Song Dialects in Alpine-breeding Songbirds of the Rocky Mountains

Abstract

Song dialects have been well documented among isolated bird populations. Natural fragmentation of bird populations among elevationally-restricted life zones could promote dialects in songbirds that use these habitats. I investigated dialect patterns among populations of songbird species breeding in isolated alpine life zones in the Rocky Mountains of Colorado. In 2010 and 2011, I recorded songs of Horned Larks (Eremophila alpestris), American Pipits (Anthus rubescens alticola), Wilson’s Warblers (Cardellina pusilla pileolata), White-crowned Sparrows (Zonotrichia leucophrys oriantha), and Brown-capped Rosy-finches (Leucosticte australis) across 19 sites among six mountain ranges in Colorado. I digitally recorded songs and used spectrographic cross-correlation (SPCC) to calculate time-frequency similarity coefficients from pair-wise comparisons of song phrases among birds. Dialects were considered evident in a species if mean, within-site song similarity was greater than mean similarity among songs from across all sites. I also used Spearman-rank correlation to determine if song similarity among paired sites was related to distance between sites (range: 3 –189 km). I did not find Horned Larks and Brown-capped Rosy Finches in large enough sample sizes to document song-sharing patterns. Within-site song similarity was greater than mean, across-site similarity for four of six populations in the American Pipit, two of four populations in the Wilson’s Warbler and, four of five populations in the White-crowned Sparrow in 2010. In 2011 I recorded birds at multiple sites within each of several contiguous alpine habitats in separate mountain ranges. This was done to test for decay in song similarity with distance within alpine habitat as well as among alpine isolates. In that year mean song similarity within site was greater than mean, across-site similarity for six of eight populations in the American Pipit, five of nine populations in the Wilson’s Warbler and, eight of nine populations in the White-crowned Sparrow. In 2011 song similarity within ranges (among multiple sites) was greater than similarity across all birds in two of four mountain-range populations in the American Pipit, Wilson’s Warbler, and the White-crowned Sparrow. Cluster analysis showed that White-crowned Sparrow songs were generally similar among sites in contiguous alpine habitat within mountain ranges, but there was no such clustering within American Pipit and Wilson’s Warbler populations within mountain ranges. Song similarity in the American Pipit or the Wilson’s Warbler was not related to between-site distance. Song similarity in the White-crowned Sparrow decreased with distance between sites, within contiguous alpine habitat in the data from 2011, but not between mountain ranges. My results provide evidence for the existence of dialects among American Pipit and White-crowned Sparrow populations in Colorado, though dialects in the latter species could be explained by clinal variation rather than by divergence among isolated alpine fragments.