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Coevolutionary Arms Race: Crossbills & Conifers

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By Holly Hastings

Two crossbill species exist in Alberta; the Red Crossbill (Loxia curvirostra) and White-winged crossbill (Loxia leucoptera). The name of this group of birds spotlights the magic of coevolution. “Crossbill” refers to the literal overlapping tips of their bills specialized to attain conifer seeds.1

Crossbills of Alberta: male white-winged crossbill (top left), female white-winged crossbill (bottom left), male red crossbill (top right) and female red crossbill (bottom right). Photos taken by AIWC staff and volunteers, or are available via public domain.
These magnificent little birds travel enormous distances across North America’s evergreen forests searching for their one and only food source: conifer tree seeds, found on spruces, firs, and pines.1 The behaviour of these birds is strongly tied to the abundance of conifer seeds as their resource. Unlike common bird nesting behaviour, where birds will often nest in the same area each year, crossbills will breed and nest anywhere and anytime on their hunt for coniferous seeds.1 If there are not enough seeds within a given area, crossbills will “irrupt” en masse to another location further away to search for more seeds. Only when they have found a large enough patch for the flock will they settle to breed.2 
Crossbills are highly specialized eaters, as is reflected in the evolution of their bills. Like crowbars, the bills move the scales of conifer cones to the side to let their tongues retrieve the seeds from their paper-like coatings.2

Evolution of the Crossbill

Evolution exists in two mechanisms (1) survival and (2) reproduction. In a population of crossbills that live in a coniferous forest, most will have bills that function in the same way. Let’s say that perhaps one or two have a bill that is angled slightly more to the left or the right. These slight differences arise because of gene mutation in an individual bird. Gene mutations can be advantageous (thicker fur in colder climates), neutral (different eye colors), or disadvantageous (diseases, for example). This makes up what we call genetic variation in a given population. 

Mechanism #1: Survival

The first step in the evolutionary process is for individuals to survive to adulthood. In crossbills, the shape of the bill may come in handy here. Say one crossbill has a beak that is angled in a way that makes it easy to extract the seeds from the conifer cones. That crossbill is able to stay well fed and grow. Perhaps another crossbill has a differently shaped beak and struggles to access the seeds. This second crossbill might not make it to adulthood.

Mechanism #2: Reproduction

Once a crossbill successfully reaches adulthood, they will reproduce, passing on their advantageous mutation (the shape of their bill) to their offspring. The offspring (with the same bill shape – though inheritance of a trait may not always be guaranteed), will follow the same path to adulthood and reproduce. Despite this, some offspring may still be born with bill shapes that are disadvantageous in survival. Over time, offspring with disadvantageous bill morphology are slowly removed from the population, as they are unable to easily access their food sources and often will not survive.

This mechanism, which may act over millions of years and generations, leads to evolution! Evolution works with what already exists. The genetic variation present in a population and the individuals that are able to pass down advantageous mutations contribute to the morphological changes in crossbills throughout history.

Coevolution

Though crossbill birds have evolved specialized bills to feed on conifer seeds, there is another factor that influences their adaptation. The coniferous trees themselves have their own defenses against predation by this guild of birds. This has led to a phenomenon between tree and bird known as coevolution. A major example of this evolutionary event has occurred between crossbills and Rocky Mountain lodgepole pine (Pinus contorta latifolia). In areas where crossbills are the main seed predator, the tree cones themselves have evolved thicker distal scales to ward off crossbills. This defense, however, influences the evolution of crossbill beak shapes in response, favouring larger beaks that can access the seeds. This back and forth, counter-offence relationship between conifers and crossbills over generations creates a kind of coevolutionary arms race.3 
Lodgepole pine tree with closed cones. Photo available via the U.S. National Park Service Gallery, 2018.
This coevolution arms race is so powerful it has resulted in some crossbill populations diverging, ultimately leading to speciation. This means that a population of crossbills has changed so significantly over time that they have become their own, separate species and are no longer able to reproduce with members of the original species.3 New, little populations of crossbill species are formed over this tug-of-war with their food source. This is a common pattern in nature, but is much more pronounced in crossbills.

How cool is that! Now, when you see crossbills or conifers out in the wild, you’ll know about this magical relationship happening just before our eyes. Isn’t nature spectacular?

References

  1. Hack, Benjamin. “Craig Benkman’s 40-Year Fascination With Crossbills and Conifers.” All About Birds. October 1 2025. Craig Benkman’s 40-Year Fascination With Crossbills and Conifers | All About Birds
  2. Centanni, Caleb., & Young, Matthew. “A Crossbill’s Guide to Conifers in Western North America: A How-to for Finding Crossbills.” Finch Research Network. July 4 2025. A Crossbill’s Guide to Conifers in Western North America: A How-to for Finding Crossbills – FINCH RESEARCH NETWORK
  3. Benkman, Craig.W. “Diversifying Coevolution between Crossbills and Conifers.” Evo Edu Outreach 3, 47–53 (2010). https://doi.org/10.1007/s12052-009-0190-8

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