LINKED PAPER Evaluating the association between vocalizations and colouration in finches (Fringillidae), a passerine group with elaborate signals. Casale, A.I., et al. (2026) IBIS.VIEW
In evolutionary biology, the interaction between multiple secondary sexual characters—like bright feathers and beautiful songs—remains a puzzle. Do species face an evolutionary trade-off?
From a theoretical point of view, three scenarios exist. Some predict a positive correlation, where multiple ornaments redundantly signal an individual’s overall quality (Møller & Pomiankowski, 1993). Others, dating back to Darwin and later formalized as the ‘transfer hypothesis’ (Gilliard, 1956), suggest a negative correlation: investing in one costly signal means less energy is available for another. Finally, if traits evolve independently, no correlation is expected, a pattern recently observed in several other avian groups (e.g., Mason et al., 2014).
Figure 1. A Red Crossbill (Loxia curvirostra) photographed in Estes Park, Colorado. This is one of the many species which was assessed for visual and acoustic traits across the diverse Fringillidae family © Agustin I. Casale.
To test this in finches (Fringillidae), a family renowned for both their elaborate plumages and their impressive singing abilities, we decided to take a two-scale approach. We looked at the family level across Fringillidae, and then zoomed in for a more detailed analysis within the genera Crithagra and Spinus. We quantified colour elaboration by measuring plumage colour patches and their contrast—specifically, by calculating a Colour Elaboration Index (CEI) based on the number of distinct colour patches in male plumage multiplied by the visual contrast between adjacent patches. We then compared this against three key aspects of vocal elaboration: energy investment, song complexity, and vocal performance (assessed by measuring ‘vocal deviation’, which indicates how closely a bird pushes its vocal apparatus to its physiological limits, given the biomechanical trade-off between how fast it sings a trill and the maximum frequency bandwidth it can produce).
Independent Paths of Evolution
Our results were clear: we found a complete absence of association between colour elaboration and all three aspects of song elaboration, both at the broader family level and within the specific genera.
Figure 2. Proportion of species pairs across Fringillidae showing no significant correlation between colour elaboration and song elaboration. The dashed red line at 0.50 indicates the expected proportion if traits are independent. From Casale et al. 2026.
Our findings align with what is becoming the most frequent result in avian research: visual and acoustic signals often evolve along independent trajectories, responding to different selective pressures or environmental constraints rather than trading off against one another.
Vocal Masters and the Complexity of Spinus Songs
While the lack of correlation was our primary finding, conducting this family-wide bioacoustic analysis confirmed that finches are true vocal masters. When we compared their trills to those of other passerine families, we found that finches can force their vocal system to sustain remarkably larger bandwidths (implying a large frequency modulation) during fast trill production.
Figure 3. Finches can force their vocal system during trill production, achieving considerably larger bandwidths in their fast trills compared to families like Emberizidae and Parulidae. In plot A, only trills up to 50 Hz were included. Plots B and C were modified from Podos (1997) and Cardoso & Hu (2011), respectively. Adapted from Casale et al. (2026).
Digging into this data further emphasized the sheer complexity of the Spinus genus. In particular, the songs of the Hooded Siskin (Spinus magellanicus) consist of a dizzying array of rapidly delivered syllables, short trills, and broadband notes that push the limits of avian vocal performance, often delivered over long periods and interspersed with repeated phrases (Casale & Clement, 2026).
An Integrated Approach to Spinus Evolution
Currently, we are building upon these findings by studying both vocalizations and coloration in an integrated manner, alongside other sources of information, to better understand the evolutionary history of Neotropical Spinus siskins in the Andes.
These ongoing studies show that the comparison of different data types reveals not only that coloration and song appear to have evolved independently, but also that there are complex patterns of evolution at multiple levels. This includes both the nuclear and mitochondrial genomes, revealing a substantial level of gene flow and introgression among Spinus species. However, these more recent results are material for another story about this fascinating group with a complex evolutionary history that is still unfolding.
References
Cardoso, G.C. & Hu, Y. 2011. Birdsong melody and the evolution of sexual dimorphism. The American Naturalist 178(2):677–689.VIEW
Casale, A. I. & P. Clement 2026. Hooded Siskin (Spinus magellanicus), version 2.0.. In: Sly, N.D. & Smith, M.G.(eds.), Birds of the World. Cornell Lab of Ornithology, Ithaca.
Gilliard, E.T. 1956. Bower ornamentation versus plumage characters in bower-birds. Auk 73(2):450-451.
Mason, N.A., Shultz, A.J. & Burns, K.J. 2014. Elaborate visual and acoustic signals evolve independently in a large, phenotypically diverse radiation of songbirds. Proceedings of the Royal Society B: Biological Sciences 281(2):20140967.VIEW
Moller, A. P., & Pomiankowski, A. 1993. Why have birds got multiple sexual ornaments? Behavioral ecology and sociobiology 32(3):167-176.VIEW
Podos, J. 1997. A performance constraint on the evolution of trilled vocalizations in bird song. Nature 390:161–163.VIEW
Image credit
Top right and featured image: Black Siskin (Spinus atratus) © Agustin I Casale.
