Gravity Gives These Birds the Drop on Tough-to-Crack Foods

Thud!

The sudden noise filled the tranquil valley in the Spanish Pyrenees. Antoni Margalida quickly scanned the sky for the source of the sound. As a field biologist for the Catalonian government familiar with the region’s wildlife, he had a hunch. And then, through his telescope, there it was: a burly Bearded Vulture, or Lammergeier, soaring with spread wings and a full goat carcass hanging from its talons. Within seconds, the vulture dropped the body again. 

Thud!

The goat smashed onto a large, flat rock. The vulture, following close behind, swooped away with the contorted set of bones and raced to the sky again. Margalida witnessed this gruesome sequence repeated at least five times before the skeleton came loose enough that the vulture could make off with a tattered, marrow-filled bone segment. Later, the bird would feed pieces of the shattered bone to nestlings, whose acid-secreting stomachs could digest the hard-won meal.

The encounter happened 25 years ago, but Margalida, now a scientist at Spain's Institute for Game and Wildlife Research, can still recall it in vivid detail. 

Bearded Vultures feed almost exclusively on bones, a diet made possible by their impressive digestive systems and impeccable accuracy in targeting bone-smashing surfaces. They are hands-down masters of this food-dropping behavior, but they’re not the only birds to employ the tactic.

At least 23 other species including gulls, crows, and eagles take advantage of rocks and pavement to crack into nuts, mollusks, and other hard-shelled food. It’s a clever technique birds use to extract otherwise inaccessible calories, and one that involves more sophistication than might be apparent.

Western Gulls, for example, tailor their technique for dropping Washington clams based on how hard each clam is to crack. The mollusk’s thick shell encloses a soft-bodied invertebrate that can't be pried loose by merely pecking the exterior. “The gulls really can’t do anything with them until they break the shell,” says John Maron, a wildlife biologist at the University of Montana.

In the early 1980s, Maron conducted a field experiment at California’s Bodega Bay to better understand how Western Gulls were able to access the hidden morsels within. He labeled clams of various sizes and weights and used those identifiers to match each mollusk’s fate with its physical characteristics. Day after day, Maron watched gulls swoop to the shore, pick up the clams he’d placed there, and then drop them—mostly from high above onto the mudflat below, but occasionally on a nearby paved parking lot and from a much lower height. His observations revealed that the birds behaved differently according to the size of the clams. The gulls would drop the lightest clams on the mudflats, where they would easily crack open from a greater height, and they took the heaviest, hardest-to-crack clams to the parking lot, where the paved surface allowed them to drop the shells from a lower height and save energy. 

Northwestern Crows, on the other hand, don't need a variety of shell-smashing sites because they only choose the biggest mollusks. In 1977, zoologist Reto Zach watched five crow pairs on Canada’s Mandarte Island carefully investigate different-size whelks, a type of sea snail. The birds would pick them up with their beaks and immediately reject the lighter and shorter individuals. That selectivity makes sense; not only do large whelks contain the most food, but Zach's research also found that, unlike with clams, the largest individuals were the easiest to break open on a rock. Once a crow found a suitably large whelk, it flew the mollusk to its beach territory, and dropped it on a designated rocky spot littered with broken shells. 

They aren’t the only corvids that put gravity to use at mealtime. In California, some city-dwelling American Crows have come to rely on roads to shatter walnut shells, according to Daniel Cristol, a behavioral ecologist at the College of William and Mary. Cristol has observed flocks of 30 to 40 crows swarming roadside English walnut trees in Davis during peak fruiting season and using the pavement below to unlock a feast. The crows appeared to seek a sweet spot for each drop, letting go of a nut from high enough to break the shell, but not so high that another bird could steal the cracked nut before they can reach it.