Causes of the latitudinal gradient in birdsong complexity assessed from geographical variation within two Himalayan warbler species

Abstract

Aspects of birdsong complexity, such as the number of distinct notes in a song, commonly increase along latitudinal gradients, a pattern for which at least 10 explanations have been suggested. In two Himalayan warblers, songs are more complex in the northwest than in the southeast. In Grey‐hooded Warbler Phylloscopus xanthoschistos, high complexity results from increased note diversity within song types, sung across a higher bandwidth. In Blyth's Leaf Warbler Phylloscopus reguloides, high complexity is a consequence of increased variation between song types. The hypothesis with strongest support is that songs evolved to be more complex in species‐poor, demonstrably less noisy environments. We consider geographical variation to be an outcome of sexual selection favouring complexity across environments, where detection of the signal varies. Sexual selection favouring complexity may be resolved in different ways, because complexity has multiple features (repertoire size, song switching, etc.). We argue this has led to the great diversity in song that we have documented among five Phylloscopus species.

The relationship between latitude, migration and the evolution of bird song complexity

“…There are both fewer bird species (Botero et al 2014, Weir & Lawson 2015 and less background noise from insects (Weir et al 2012) at higher latitudes, potentially freeing bird song to evolve more complex forms. Irwin (2000), Singh and Price (2015) and Wei et al (2017) suggest latitudinal variation in sound space may explain some or most of the variation in song form detected in their respective studies.…”

“…While many studies do attempt to take the environment into account in some way (such as by classifying them as 'boreal' vs. 'tropical' forest or 'open' vs. 'closed' species), only two studies included here actually measured the habitat and acoustic features at their study sites (Irwin 2000, Singh & Price 2015, and another three used mean climate measures (Botero et al 2009, Medina & Francis 2012, Xing et al 2017. Fully to test the ecological hypotheses included in this review the field requires more studies that explicitly quantify the sound space available at varying latitudes and to determine whether more complex songs fill a wider swathe of that space.…”

Distinct song parts of the endemic marsh grassbird of China vary with latitude and climate among migratory and sedentary populations

“…However, it should be no surprise that climate and latitude together may better predict variation in sexual selection pressures related to variable and unpredictable climate (Irwin 2000;Botero et al 2009). The correlation between spectral bandwidth and latitude may also be attributed to habitat structure and community composition: selection against using higher frequencies may be relaxed because of more open vegetation and less competition for acoustic space due to fewer vocally active sympatric species in the north (Weir et al 2012;Singh et al 2015). Although community richness of vocal bird species and relative habitat openness in our seven populations may vary along the lines of these published studies, such measurements are not yet available and await future studies.…”

“…Sedentary populations may also have more convergent songs through copying of neighbourhood songs, at least in some species, driven by the higher consistency in the identity of neighbouring males (Kroodsma 1999). Furthermore, variation in habitat density and species diversity may yield variable competition for acoustic space, and thereby allow song elaboration more easily at relatively high latitudes, where vegetation is typically less dense and acoustic competition with other species less intense (Weir and Wheatcroft 2011;Singh et al 2015). Several comparative studies have confirmed positive correlations between song elaboration and latitude, both among (Catchpole 1982;Botero et al 2009) and within species (Collins et al 2009;Kaluthota et al 2016).…”

The evolutionary origin of variation in song length and frequency in the avian family Cettiidae

“…In comparisons across selected species, song frequency is regularly negatively correlated with body size (Wallschläger 1980, Linhart andFuchs 2015), as well as affected by habitat openness and ambient noise (Boncoraglio andSaino 2007, Luther andBaptista 2010). A second correlate of songs is that complexity and/or length regularly increases with latitude (Irwin 2000, Singh and Price 2015, Kaluthota et al 2016). A second correlate of songs is that complexity and/or length regularly increases with latitude (Irwin 2000, Singh and Price 2015, Kaluthota et al 2016).…”