Crossbills: Protagonists in an Evolutionary Arms Race
By Maja Mielke
It was already getting dark as I wandered through the forest, and I hadn’t seen another person for the past 30 minutes. I knew I should head home to make it back before sunset, but I wasn’t ready to give up just yet. Holding my binoculars firmly in my hand, I gazed at the tips of the conifers across the creek. Only gray silhouettes before the darkening November sky. Hundreds of cones hung from the branches of the trees. This forest seemed like the perfect habitat for the bird I was searching for: the Red crossbill.
Red crossbill. Photo by Daniil Komov from Pexels.
I’m a biologist conducting research in functional morphology, studying the biomechanics of beak movement in feeding songbirds. I can’t help but be fascinated by the unique morphology of beaks with crossed beak tips. Among the more than 10,000 bird species in the world, the six species of crossbills are the only ones with crossed beak tips. These beaks allow them to access a type of food that is hardly accessible to any other bird: the seeds hidden deep inside conifer cones, protected by tough cone scales.
Right-beaked or left-beaked?
Have you ever wondered whether the crossbills’ beaks are always crossed in the same direction? The answer is no. Indeed, the ratio of left- and right-crossed beaks is approximately 1:1. Surprisingly, researchers have not yet figured out what exactly determines the crossing direction of the beak. Although a genetic basis for mandible crossing seems likely, researchers have not found a direct link between genetics and crossing direction. They observed that closely related individuals within a population are not more likely to have the same crossing direction. That means two birds with right-crossed beaks won’t necessarily have offspring with primarily right-crossed beaks. Researchers concluded that crossing direction, if at all, has a very low heritability.
A unique and highly efficient feeding technique
How does a crossbill extract the seeds from closed conifer cones within the blink of an eye? It’s simple and astounding at the same time. First, the bird inserts the beak sideways under the cone scale, so that the tip of the lower beak faces towards the cone axis. Then, they open their lower beak sideways and twist the head, using the mandible as a lever. In doing so, they create a gap large enough for the tongue to extract the seeds hidden beneath the scale. This feeding technique is highly efficient, and crossbills feed on up to 20 seeds per minute and up to 3,000(!) seeds a day. If you wish to see this remarkable behavior in action, I recommend taking a look at this video from the Cornell Lab of Ornithology: watch here.
The sideways movement of the lower mandible is an essential part of the success in feeding on cone seeds. However, the ability to move the lower beak sideways is not exclusive to crossbills. Birds have a highly kinetic skull, which enables most of them to move their lower mandible sideways at least to some extent. For instance, in my PhD research, I showed that canaries rely on this extra degree of freedom when feeding on hemp seeds. Does that mean that canaries could open a cone scale as well? No. The crossing of the beak tips in crossbills is what makes the difference. Without the crossed beak tips, birds cannot open the scale sufficiently wide to reach the seed. Crossbills whose beak crossings had been removed couldn’t feed on cone seeds anymore, but they regained that ability after the beak tips regrew. Additionally, the crossbills’ jaw musculature is specialized for that feeding technique: the jaw muscles on the side to which the lower mandible bends are enlarged. This allows the birds to produce lateral forces that other songbirds could never achieve. It is a remarkable and unique tool set that makes the crossbills true specialists for their diet. A tool set that still continues to evolve!
An evolutionary arms race
If a single crossbill feeds on up to 3,000 seeds a day, the impact of a whole crossbill population on conifer reproduction can be immense. However, the trees know how to defend themselves. Conifers such as pines vary in the thickness of their cone scales. Some seeds may be so heavily protected that even the most adept crossbills cannot reach them. If mostly these seeds are left behind, the cones of the next pine generations will have thick cone scales as well. Only the crossbills with the strongest beaks will be able to open them. This, in turn, favors strong bills in the crossbill population, rendering the thick cone scales more ineffective again. It’s a textbook example of an evolutionary arms race!
My personal endeavor continues
Seeing a crossbill in the wild has long been at the top of my birding wish list. However, on that November evening in the conifer forest, I ultimately didn’t find one. Maybe it was the wrong year. Crossbills are nomadic, migrating between pine forests depending on the abundance of cones. But I won’t give up. I’ll keep searching through conifer forests for this beautiful bird with its extraordinary beak. I’m determined to see that beak in action someday.