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# Beak Length Changed Faster Than Natural Selection Allows
- URL: https://www.gottabekidding.me/beak-length-changed-faster-than-natural-selection-allows/
- Published: 2026-10-15T10:02:00.000Z
- Updated: 2026-10-15T10:01:59.000Z
- Description: Beaks on one campus swung up and back at a pace near the top of 2,400 documented cases, yet no population crash was reported. What moved them?
- Author: Field Notes
- Tags: Nature

Dark-eyed juncos (Junco hyemalis) on the UCLA campus hatched with longer beaks, on average, in 2021\. By 2023 the average had swung back down. If natural selection alone were responsible, the pace would sit near the top of roughly 2,400 recorded cases of wildlife responding to human disturbance.

Diamant and Yeh tracked campus juncos and non-urban juncos from 2018 through 2025, and second-year campus birds mostly had shorter beaks. They proposed that pandemic lockdowns thinned the supply of discarded human food, and that beaks adapted to a changed menu.

A letter in PNAS from Samson Acoca-Pidolle and Laurine Mathieu doubts it. They calculated how fast evolution would have to run to produce the swings, assuming adaptation explained everything. The answer was 0.76 haldanes for 2021 and negative 0.86 for 2023, a haldane being a measure of evolutionary speed. That would rank 2021 third and 2023 second among them.

For scale, Darwin's finches (Geospiza fortis) shifted beak length by 0.65 haldanes over two years of drought in the 1970s. In that stretch 85 percent of the birds alive in 1976 died or left, and nobody bred in 1977\. A junco shift that large would demand major losses in nesting or survival, the authors reason. No population crash was reported.

So what moved the beaks? The letter offers candidates, not answers. Beaks may develop differently under different conditions. Outside birds may have moved onto campus more freely as human pressure eased. Adult beaks were already lengthening in 2021, in birds hatched before lockdowns. Beak length may also ride along with other traits, such as stress reactions or brain size.

Darwin's finches needed a drought that erased most of the flock to move their beaks this far. The UCLA juncos covered more ground, and nobody reported a missing flockmate.

**Read the full story at** [PNAS, September 28, 2026](https://www.pnas.org/doi/abs/10.1073/pnas.2621014123?af=R&ref=gottabekidding.me)

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**Hot Take:** Darwin's finches lost 85 percent of their flock to shift their beaks. No missing juncos have been reported, only a missing explanation.