Every year around the end of winter, baby spotted nutcrackers peck their way out of their shells, ready to learn as much as they can from their parents about how to live as a bird—such as how to bury seeds throughout the year for later consumption. Spotted nutcrackers are fairly unique, even among seed-caching birds, because they rely on the seeds from just one kind of tree: the Swiss stone pine trees of the Carpathian Mountains and the Alps. Reciprocally, the tree relies on them for sowing seeds.
The problem for the birds is that these seeds are available for harvest only between August and October. If the nutcrackers are not careful, the seeds they bury could easily germinate and become saplings, making the deposit useless as a meal weeks or months later. Thus, spotted nutcrackers have a clever greengrocer system, according to a new study: they choose food-hiding places that maximize the shelf life of seeds, where perishability is lowest.
Most caching birds choose their hiding spots carefully, but no one has known exactly how. The prevailing hypothesis has been that the birds decide in favor of sites that minimize the likelihood that other animals will discover their caches and pilfer them. Eike Lena Neuschulz, a postdoctoral researcher at the LOEWE Biodiversity and Climate Research Center in Frankfurt, Germany, recently tested that assumption. She and her colleagues randomly cached 900 stone pine seeds at sites within one of five microhabitats in the eastern Swiss Alps, such as under snow, beneath a tree or near shrubs. Then the researchers spent nearly 400 hours watching spotted nutcrackers hide their own caches.
By comparing their experimental caches with those of the birds, Neuschulz discovered that the nutcrackers explicitly chose to hide seeds in the areas that were least likely to sprout a tree—sites that lacked the requisite soil moisture or were heavily shaded. The team also found that all caches were equally likely to be pilfered, no matter their surroundings, according to results published in January in the Journal of Animal Ecology.
“The Swiss stone pine can [live] more than 500 years,” Neuschulz says, “so it doesn't need a lot of successful germination events to maintain population viability.” That life cycle explains how these evergreens evolved to rely on birds that attempt to hide the seeds where they are least likely to result in a new tree. Just a few germinated seeds each season spread out over multiple centuries can yield quite a lot of offspring. Meanwhile the spotted nutcrackers have learned to use nature as a breadbox, keeping their food from spoiling.