Black pine cone crop size, cone structure and pollination failure affect the amount of seed predation by crossbills in the Iberian Peninsula.
Mezquida ET, Benkman CW.
Seed Saving
Next time you spot a crossbill working through a pine cone in a mountain forest, you're watching evolution in slow motion — the bird's beak shape and the cone's thickness are locked in an arms race playing out across centuries.
Crossbills are birds with beaks specially shaped to pry open pine cones and eat the seeds. Researchers found that crossbills consistently pick trees with lots of small cones, which should pressure pines to grow bigger, tougher cones as a defense over generations. But across different forest sites in Spain, how much damage crossbills actually do depends more on whether the pine's seeds were properly pollinated that year — often tied to rainfall — than on cone shape itself.
Key Findings
Crossbills preferentially targeted trees producing many small cones with thin scales, a consistent preference pattern matching findings across multiple conifer species worldwide.
Across nine Iberian Peninsula sites, the proportion of empty (unfertilized) seeds — not cone structure — was the strongest negative predictor of crossbill seed predation intensity at the site level.
Empty seed rates appear driven by year-to-year environmental variation (e.g., precipitation affecting pollination success), meaning site-level differences in predation pressure are unlikely to produce stable geographic differences in cone evolution.
chevron_right Technical Summary
Crossbills prefer to raid small, thin-scaled black pine cones, which could push pines to evolve bigger, tougher cones over time — but this evolutionary pressure varies wildly between forest sites depending on how many seeds were pollinated that year.
Abstract Preview
Understanding the causes of geographic variation in traits is the focus of much research. Prior studies on multiple species of cone-bearing conifers have discovered that geographic variation in con...
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