Diversification in trophic morphology and a mating signal are coupled in the early stages of sympatric divergence in crossbills

Abstract

Understanding the mechanisms generating diversity in mating signals is critical to understanding the process of speciation. One mechanism of mating signal diversification occurs when phenotypes that experience divergent ecological selection also affect the production of mating signals, resulting in a coupling between ecological diversification and mating signal diversification. Here, we present evidence that rapid diversification in bill size has resulted in the diversification of some components of song structure in a young adaptive radiation of seed-eating finches (red crossbill, Loxia curvirostra complex). Specifically, we find that larger-billed ecotypes sing songs with lower minimum frequencies, lower syllable repetition rates and greater vocal deviation (i.e. lower performance) than smaller-billed ecotypes for pure tonal syllables. In contrast, bill size was not correlated with maximum frequency or frequency bandwidth, and we found no relationship between bill size and any song parameters in buzzy syllables. Furthermore, we found no evidence for a relationship between the degree of bill size divergence and the potential for song discrimination between sympatric ecotypes. Because bill size is correlated with some features of pure tonal syllables (which appear to be most important for courtship in crossbills) in crossbill song, our results suggest that there was an early-evolving link between ecological and mating signal diversification that may have influenced the rapid evolution of reproductive isolation between sympatric ecotypes.

Relationships of song structure to phylogenetic history, habitat, and morphology in the vireos, greenlets, and allies (Passeriformes: Vireonidae)

“…A positive correlation between bill morphology and avian song traits also occurred in woodcreepers (Furnariidae; Palacios and Tubaro 2000), Darwin's finches (Podos 2001), and true finches (Fringillidae; Giraudeau et al. 2014; Porter and Smith 2020). Birds with larger bills cannot manipulate their gape size with the height necessary for higher frequencies (Podos 2001).…”

Fast cultural evolution of Crossbill (Loxia spp.) calls in the Palaearctic

“…The indirect effect implies, for example, a change of food resources and a subsequent adaptation of the morphology. Morphology can affect bird songs, with bigger birds tending to produce lower frequencies and bill size limiting temporal and performance components ( Podos 2001, Christensen et al 2006, Huber & Podos 2006, Derryberry et al 2018, Porter & Smith 2019). However, birds are obviously not always at the limit of their vocal capabilities (e.g.…”

“…2006, Huber & Podos 2006, Derryberry et al . 2018, Porter & Smith 2019). However, birds are obviously not always at the limit of their vocal capabilities (e.g.…”

Response to Hill and Powers: It is irrelevant that the mode and tempo of Cassia crossbill speciation is not typical for birds

“…Cassia crossbills were categorized as one of 10 call types of red crossbill in North America numbered in sequence of discovery (call type 9 (Sibley 2000); see Groth (1993) for a description of the first eight call types and Irwin (2010) for a description of call type 10). This was based on the Cassia crossbill's distinctive contact calls and song, and larger average size compared to other co‐occurring red crossbills (Benkman et al 2009, Porter and Smith 2020). Many North American call types are strongly associated with and have bill structures adapted to feeding on the cone‐seed of particular species of conifers (Benkman 1993, 2003, 2007, Parchman and Benkman 2002, Irwin 2010).…”

“…Regardless, Snowberg and Benkman (2007) examined preference in relation to contact calls only. Cassia crossbills have distinctive songs (Porter and Smith 2020), and we strongly suspect that 1) females also prefer the song of their father, 2) song is unlikely to switch even occasionally between types and 3) song undoubtedly plays an important role in mate choice and promotes assortative pairing beyond that based on calls alone (Uy et al 2018).…”