Visualizing Science: Why are there so few white-winged crossbills?

Post by Cody Porter
Hand-drawn image of a white-winged crossbill, a bird with red body, black and white wings, and a beak that is actually

Excerpt of full illustration, © Cody Porter, 2018
​This image represents my attempt to illustrate a striking pattern in the system I study: crossbills (Loxia).

Globally, and in the Americas specifically, there is a huge discrepancy between overall and local (sympatric) diversity in the two crossbill clades (the ‘red’ and ‘white-winged’ clades).

In the Americas, there are only two forms of white-winged crossbill and they are both geographically separated by thousands of miles (one occurs in Hispaniola, the other in the boreal forest of North America). In contrast, there are 11 forms of red crossbill in the Americas, and in many regions multiple forms can be found breeding in the same forest!

The target audience for this image spans a wide range of backgrounds. I think the pattern is so striking and easy to see that individuals without much formal training in biology will be able to appreciate the discrepancy between these crossbill clades. On the other hand, individuals in my field of evolutionary ecology will (hopefully) recognize the biological puzzle these patterns pose. Looking at this image, they may generate hypotheses that could possibly explain the pattern.


This image is a spatial heat map of the distributions of white-winged and red crossbill taxa in the Americas. I’ve also included representative illustrations of white-winged and red crossbills and the spectrograms of flight calls made by the different taxa. Flight calls are the only reliable way to identify many red crossbill taxa in the field, and appear to have played an important role in their diversification. Image credit: © Cody Porter, 2018
I chose to represent the discrepancy in local and regional diversity between white-winged and red crossbills by creating a spatial heat map, wherein I’ve overlaid the distributions of crossbill forms onto a map of North and Central America. Regions with higher local diversity are represented with warmer colors, while regions with lower local diversity are represented with cooler colors. I’ve also included representative illustrations of red and white-winged crossbills. Finally, I’ve added spectrograms (visual representations of sound – time on the x axis and frequency of the sound on the y axis) of each crossbill, along with their common name. This image was made using watercolor paint.

I had two main images that inspired this project. One, is an early global range map of red and white-winged crossbill forms in Ian Newton’s 1972 book, Finches. The other images are spatial heat maps depicting spatial variation in the escalation of a coevolutionary arms race between a predator (garter snakes) and its prey (newts). These latter images have been published in several scientific papers by Edmund Brodie’s group at the University of Virginia. The next step for this image is to create a digital version, wherein I use ArcMap to create the spatial heat maps.


This image is from Ian Newton’s book titled Finches. This image depicts the geographic distributions of multiple red and white-winged crossbill taxa, though it is incomplete, and many additional taxa were described after this book was published. Source: Photo of Newton (1972) Finches

This is an image on a page of the journal Evolution from an article by Ed Brodie’s group. The spatial map here is depicting geographic variation in the degree of resistance of garter snakes to a toxin produced by their newt prey, with warmer colors indicating regions of higher resistance. Source: screenshot from Brodie et al. (2002) Evolution

This is a similar image from a page of the journal PLOS Biology, also from Ed Brodie’s group. This spatial heat map depicts variation in the extent to which the traits of garter snakes and newts are most mismatched (termed coevolutionary hotspots). Source: Screenshot from Hanifin et al. (2008) PLOS Biology

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