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Size and Weight Help Wild Carrion Crows Choose Walnuts

Research: Behaviour, Ecology

Wild Carrion Crows (Corvus corone) can use a walnut’s visible size and information gained by handling it when choosing food. In field experiments in Japan, researchers offered wild crows pairs of walnuts. The shell hides the edible kernel, and cracking it takes time and effort, so choosing a nut likely to contain more food can matter.

Size influenced the first choice

In 241 trials with differently sized walnuts, crows first touched the larger nut in 66.8% of trials and ultimately retrieved it in 80.9%. The larger walnut was therefore more likely to attract their initial attention and to be the one they took. The greater the weight difference between the pair relative to the lighter nut, the more likely the crows were to retrieve the larger one.

Handling helped reveal what was inside

In a second test, the walnuts were similar in size, but one contained a kernel and the other was empty. Across 80 trials, crows showed no clear preference for which nut they touched first, yet retrieved the filled walnut in 75% of trials. They switched to the other nut more often after first touching an empty one: this happened in 60.6% of those trials, compared with 34.0% after a filled walnut was touched first.

Lifting was also associated with the final choice. In the size test, lifting a walnut tended to reinforce its selection, whether it was the larger or smaller one. In the content test, lifting the filled walnut was associated with choosing it, while lifting the empty one was not. The findings suggest that the crows could refine an initial visual choice by handling the nuts and gaining information related to their mass. The experiments did not identify the exact sensory cues involved.

Across both tests, crows were more likely to choose the higher-quality option—the larger or filled nut—when the weight difference between the two walnuts was greater relative to the lighter one. The results show how wild crows can combine what they see with information gained while inspecting potential food.

Sources and image credit

  1. Wild Carrion Crows Choose Walnuts Based on Visual and Weight CuesPrimærstudie. Publisherens fulltekst ble brukt til å kontrollere forsøksoppsett, resultater og forfatternes tolkning.
  2. File:Corvus corone R097 walnut 20181027 t104748.jpgBildekilde. Filen viser en kråke som spiser en valnøtt; kreditering og lisens er oppgitt på filsiden.

Image: Marie-Lan Taÿ Pamart / Wikimedia Commons, CC BY 4.0. Representative photograph. · CC BY 4.0 · Source

About this content: This story was produced with AI assistance within an editorial workflow developed by Wildlife Vagabond. Editorial responsibility remains with Wildlife Vagabond.How AI is used

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