Geographic Variation in Type I Songs of Black-throated Gray Warblers

Abstract

We studied songs of Black-throated Gray Warblers (Dendroica nigrescens) in a fragmented landscape in southwestern Oregon and northern California where each male sings a single Type I song consisting of two phrases. Fourteen variants of Type I songs were distributed in a complex geographic pattern across 19,400 km2 of the region. Variants differed in number of notes/syllables in the A-phrase (range 2–5), and the B-phrase differed in both the number and structure of syllables. Several variants occurred in well-defined areas and differed from neighboring songs; others overlapped adjacent variants or graded from one form to another across a narrow zone. Distinct variants could be identified, but the diversity of Type I songs and the pattern of distribution throughout the region does not describe a clear system of dialects. Geographic extent of the variants differed considerably; some occurred as small scattered populations occupying <250 km2 while the largest exceeded 3,000 km2. Variants in the most restricted area and having the most fragmented distribution had the least consistent structure among individuals both within local populations and across the range of the variant. Ridges >1,000–1,200 m in elevation served as effective barriers and separated sets of similar song variants. Fire also likely had a role in generation of variants as reflected by multiple variants occurring in areas lacking obvious geographic barriers.