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Red Crossbills Preferred Small Black Pine Cones, but Seed Predation Was Lower Where More Seeds Were Empty

Female red crossbill perched in natural habitat in Serra da Arrábida, Portugal
Research · Evolution & taxonomy

A study at nine sites on the Iberian Peninsula asked whether differences in seed predation by red crossbills (Loxia curvirostra) could explain geographic variation in the cones of black pine (Pinus nigra). The researchers compared crossbill feeding preferences within sites with differences in cone traits and pre-dispersal seed predation among sites.

Crossbills favoured small, thin-scaled cones

Within individual sites, crossbills preferentially foraged on trees with large cone crops and on small cones with relatively small, thin scales. The pattern was similar to the feeding preferences reported in earlier studies of crossbills and cone-bearing conifers.

Because cone size and scale thickness are often heritable, the authors argue that repeated preference for small, thin-scaled cones could impose natural selection favouring larger cones with thicker scales. The study therefore identifies a feeding preference with the potential to drive evolutionary change within populations; it does not by itself show that such change has already occurred.

Empty seeds complicated the geographic pattern

The comparison among the nine sites produced a different result. Overall crossbill seed predation tended to be lower where a larger proportion of the seeds were empty. The authors link empty seeds to pollination failure, which can in turn vary with environmental conditions such as precipitation.

That matters because pollination conditions can change from year to year. The authors therefore conclude that geographic differences in predation intensity are unlikely, on their own, to produce matching geographic differences in cone traits. Some of the apparent variation in predation among sites may instead reflect changing availability of filled seeds.

The broader point is methodological as well as evolutionary: to test whether seed predators explain geographic variation in defensive cone traits, researchers need to separate predator-driven selection from environmental variation that changes how many usable seeds are available. In this study, crossbill preferences within sites were consistent with directional selection on cone structure, while the among-site pattern was complicated by variation in empty seeds that the authors link to pollination failure.

Sources and image credit

  1. Mezquida, E. T., & Benkman, C. W. (2026). Black pine cone crop size, cone structure and pollination failure affect the amount of seed predation by crossbills in the Iberian Peninsula. Plant Biology, 28(4), 1237–1244. https://doi.org/10.1111/plb.70173Primary peer-reviewed research article; first published online 5 January 2026.

Image: Luiz Lapa — female red crossbill in Serra da Arrábida, Setúbal, Portugal, photographed 18 November 2021; representative Iberian species image. Source: Wikimedia Commons. · CC BY 2.0 · Source

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Evolution & taxonomy

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