Transcontinental latitudinal variation in song performance and complexity in house wrens (Troglodytes aedon)

There is growing interest in latitudinal effects on animal behaviour and life history. One recent focus is on birdsong, which is hypothesized to be more elaborated or complex in the north temperate zone compared with the tropics. Current evidence is mixed and based on cross-species comparisons, or single species with restricted distributions. We circumvent these limitations using a transcontinental sample of 358 songs from house wrens (Troglodytes aedon) at 281 locations spanning more than 100° of latitude (52° N–55° S) across the Americas. We found a significant latitudinal gradient in several basic elements of song performance and complexity between north temperate and tropical populations. Furthermore, we document convergence in song patterns between populations at higher latitudes in the Northern and Southern Hemispheres. Effects were strongest for the number of elements in a song, and the rate of element production, both increasing towards the poles, with similar but weaker effects for other song dimensions (e.g. number of unique elements, trills and trill rate). We consider possible causes related to variable habitats and morphology, concluding that the shorter breeding seasons at higher latitudes in both hemispheres may favour greater song elaboration to mediate territory competition and mate choice.     

Bud phenology and growth are subject to divergent selection across a latitudinal gradient in Populus angustifolia and impact adaptation across the distributional range and associated arthropods

Temperate forest tree species that span large geographical areas and climatic gradients often have high levels of genetic variation. Such species are ideal for testing how neutral demographic factors and climate‐driven selection structure genetic variation within species, and how this genetic variation can affect ecological communities. Here, we quantified genetic variation in vegetative phenology and growth traits in narrowleaf cottonwood, Populus angustifolia, using three common gardens planted with genotypes originating from source populations spanning the species' range along the Rocky Mountains of North America (ca. 1700 km). We present three main findings. First, we found strong evidence of divergent selection (Q_ST > F_ST) on fall phenology (bud set) with adaptive consequences for frost avoidance. We also found evidence for selection on bud flush duration, tree height, and basal diameter, resulting in population differentiation. Second, we found strong associations with climate variables that were strongly correlated with latitude of origin. More strongly differentiated traits also showed stronger climate correlations, which emphasizes the role that climate has played in divergent selection throughout the range. We found population × garden interaction effects; for some traits, this accounted for more of the variance than either factor alone. Tree height was influenced by the difference in climate of the source and garden locations and declined with increasing transfer distance. Third, growth traits were correlated with dependent arthropod community diversity metrics. Synthesis. Overall, we conclude that climate has influenced genetic variation and structure in phenology and growth traits and leads to local adaptation in P. angustifolia, which can then impact dependent arthropod species. Importantly, relocation of genotypes far northward or southward often resulted in poor growth, likely due to a phenological mismatch with photoperiod, the proximate cue for fall growth cessation. Genotypes moved too far southward suffer from early growth cessation, whereas those moved too far northward are prone to fall frost and winter dieback. In the face of current and forecasted climate change, habitat restoration, forestry, and tree breeding efforts should utilize these findings to better match latitudinal and climatic source environments with management locations for optimal future outcomes.     

Conventional and network analyses of song organization and complexity in northern House Wrens (Troglodytes aedon parkmanii)

Among songbirds with large song-type repertoires, there may be functional variation in how individuals organize and display song-type diversity over time. Past studies focusing on conventional measures of song production have been extremely productive. However, network analysis offers a novel set of tools to quantify additional, previously unstudied elements of song organization and display. We studied protracted bouts of singing by 10 male House Wrens (Troglodytes aedon) to (1) test functional hypotheses of variation in song diversity in this species, and (2) evaluate the utility of network metrics in such research. Our analysis included a variety of conventional measures of song production and several standard metrics from network theory to quantify how variably the many song types in a male’s repertoire could be connected to one another and the limitations or diversity of their song sequences. Analysis of conventional variables showed that males produced more and longer songs, at shorter intervals, containing more syllables and more different syllable types, and also more different song types, prior to than after pairing and early in the morning regardless of breeding stage. These results are consistent with the hypothesis that song diversity functions in mate attraction and possibly in territory signaling. In contrast, analyses of network metrics revealed variety in song sequencing by males, but comparatively few and weak effects associated with either breeding stage or time of day. Overall, most song types connected to only a few others and a relatively small proportion of all possible song-type transitions actually occurred. Hence, much of the variety in song sequencing that was possible with the large song repertoires of males was not realized. The latter outcomes, brought to light via network analyses, highlight an important paradox for future research on this and related species with large song repertoires.