Birds have evolved a variety of techniques to open hard foods like nuts, seeds, shellfish, and bones in order to access the nutritious contents inside. While some birds swallow small hard objects whole and rely on their muscular gizzard to grind up food, other birds have adapted more elaborate ways to break open hard-shelled food sources before swallowing. One interesting technique employed by certain bird species is dropping bones or shellfish from heights onto hard surfaces below in order to crack them open through the impact.
This behavior of actively dropping food objects to break them open is known as “drop-smashing” and has been observed in several types of corvids – the family of birds including crows, ravens, rooks, jackdaws, jays, magpies, and nutcrackers. Research has revealed that the most prolific drop-smashers tend to be larger corvid species, such as crows and ravens. The habit of dropping food from heights enables them to apply enough force to successfully smash open otherwise hard-to-access food sources like bones, clams, mussels, oysters, snails, and crabs.
What Birds Drop-Smash Bones?
Among corvids, common ravens and American crows are especially frequent practitioners of dropping bones to shatter them. Studies monitoring raven populations in wilderness areas have recorded ravens dropping mammalian bones from soaring heights of 50 feet or higher onto flat rocks below. Analysis of the impact sites below habitual raven nesting spots have turned up collections of broken bone fragments, confirming that ravens do indeed target hard substrates when food-dropping.
In American crows, the behavior has been well-documented through field observations. Researchers in California recorded American crows dropping walnut shells from heights up to 100 feet onto paved roads and rocky stream banks to crack them open through the force of impact. Slow-motion analysis revealed that crows release the shells from their beaks just before hitting the ground to avoid any damage to their beaks or heads from the impending collision. Besides nuts and shells, American crows have also been documented dropping hard-shelled prey like clams and mussels onto hard surfaces and waiting for the shells to crack before swooping down to access the flesh.
Other Bird Species That Drop-Smash
While corvids are the most prolific avian drop-smashers, a few other distantly related bird species have independently evolved the food-dropping behavior as a way to open hard-shelled food sources:
- Lammergeiers – Also known as bearded vultures, these raptors inhabiting mountainous regions of Europe, Asia, and Africa drop bones from heights of over 160 feet onto rocks below to gain access to marrow inside.
- Egyptian vultures – This Old World vulture drops ostrich eggs and other large eggs onto rocks to crack the thick shells and feed on the yolks.
- Northwestern crows – The Northwestern crow, a species from coastal East Asia, drops shellfish from heights to crack the shells.
- Palm cockatoos – The large black cockatoos of New Guinea drop nut-bearing palm fruits from heights to split them open.
Why Do Birds Drop-Smash Food?
Birds like ravens and crows have evolved the food-dropping behavior because it allows them to make use of a wider range of hard-to-access food sources in their environments such as bones, nuts, and shellfish. Being able to extract the nutritious contents inside these objects provides an important source of calories and nutrients in birds’ diets.
Researchers have identified several advantages to drop-smashing over alternative ways of opening hard foods:
- It takes less time and effort than holding an object and manually hammering at it repeatedly with the beak.
- Dropping objects from heights generates greater forces to crack hard shells than can be achieved by hammering from the beak alone.
- Releasing objects before impact avoids damage to beaks and skulls that could result from banging them repeatedly against resistant surfaces.
- Gravity does most of the required work in drop-smashing, minimizing the birds’ energy expenditure.
Additionally, dropping food onto flat horizontal surfaces like large rocks presents a stable, wide target area that does not require precise aiming. This makes the food-dropping technique reliable and efficient. The ability to exploit nutritious and plentiful but hard-to-access food sources like bones, nuts, and shellfish through drop-smashing provides corvids and other drop-smashing species with an advantage over birds lacking this resourceful foraging behavior.
The Mechanics of Avian Food-Dropping
The force exerted on a dropped object increases with height due to acceleration from gravity. Based on calculations and field observations, most effective drop-smashing heights for various bird species fall in the range of 10 – 150 feet. At the top end of this range, a bone dropped by a raven reaches speeds upwards of 60 miles per hour before impact – easily generating enough force to shatter bone or crack hard shells.
Studies monitoring food-dropping behavior in American crows have revealed some interesting insights into the mechanics and precision of their targeting abilities:
- Crows preferentially drop objects onto hard, flat surfaces like concrete or large rocks rather than softer dirt or grass.
- They show precise aim in dropping shells onto road surfaces averaging just 20 inches wide.
- Calculations suggest crows innately compensate for elevation and distance when lining up targets from higher heights.
- They calculate the appropriate combination of height and horizontal distance to generate sufficient force for cracking different food objects based on size, mass, and hardness.
Overall, extensive field experience and trial-and-error learning allow corvids and other habitual food-dropping birds to become excellent judges of exactly how high and how far to carry food objects before release for maximum crushing efficiency.
Development of Avian Food-Dropping Behavior
The food-dropping behavior appears to be learned rather than innate in nestling corvids. Young ravens and crows typically acquire the habit by observing parents, relatives, or unrelated conspecifics drop-smashing foods successfully. Studies of raven nesting areas show that initial clumsy food-dropping attempts begin around 4 months of age. Proficiency improves progressively through extensive practice over the first 2 years, corresponding with refinement of flight skills and physical maturation.
Adult ravens seem to selectively demonstrate food-dropping to juveniles when rare, highly prized food items like bones, snails or clams are available, suggesting an element of intentional pedagogy. Independent juvenile experimentation fine-tunes the developing skill. Similar patterns of observational learning followed by practice characterize the development of proficient food-dropping abilities in young American crows. Once mastered, the food-dropping technique becomes a regular part of the foraging repertoire used throughout the bird’s lifetime whenever suitable hard foods are encountered.
Examples of Avian Food-Dropping
Here are some specific examples that illustrate how certain bird species use food-dropping to access hard-to-open food sources:
Ravens
- Common ravens inhabiting coastal regions notoriously drop thick-shelled marine mollusks like clams, mussels and oysters onto rocks from heights up to 100 feet to crack the shells and consume the meat inside.
- Ravens in wilderness settings drop carpals, tarsals and otherbones scavenged from large mammal carcasses onto flat rocks from as high 150 feet to shatter them and eat the nutritious marrow inside.
- One group of ravens in Canada was observed dropping unshelled walnuts onto a frozen lake from a height of 100 feet. The hard ice surface split the nut shells on impact, allowing the birds to retrieve and eat the nutmeats.
Crows
- American crows inhabiting coastal forests drop heavy snail shells onto rocks from heights up to 100 feet to crack them open and consume the snail inside.
- American crows living alongside California highways puncture walnuts with their beaks before selectively dropping them onto hard concrete road surfaces from heights around 100 feet to split the shells and access the nutmeats.
- Northwestern crows on Japanese seashores carry shellfish like clams, mussels and oysters aloft and drop them onto rocky intertidal areas from as high as 80 feet up to crack the shells.
Palm Cockatoos
- Palm cockatoos in rainforests of New Guinea use their massive curved beaks to peel away and break off individual coconuts from bunches at the tops of palm trees. They husk the coconuts while aloft before dropping them onto tree buttress roots from heights of over 50 feet to split open the hard inner shell and eat the flesh inside.
- These cockatoos similarly break open hard-shelled Canarium almonds growing in high rainforest canopies by detaching whole fruits and dropping them onto rocks below.
Bearded Vultures
- Bearded vultures frequenting hilly areas in Europe and Asia drop large bones scavenged from mountainous terrain onto flat, stony surfaces below rocky precipices and cliffs. Drops from heights approaching 165 feet shatter mammalian bones, allowing the vultures to consume nutritious bone marrow.
- They also drop heavy turtle shells from great heights to crack them open and access the meat inside.
Egyptian Vultures
- Egyptian vultures break open thick-walled ostrich eggs, which can weigh over 3 pounds, by dropping them onto flat rocks from as high as 100 feet up. The impact shatters the hard shell, allowing the birds to lap up the nutrient-rich yolk inside.
- They use the same food-dropping technique to break open other large eggs with thick shells like those of bustards, emus and storks.
Bird Species | Foods Dropped | Maximum Drop Height |
---|---|---|
Common Raven | Bones, shellfish | 150 feet |
American Crow | Nuts, snail shells | 100 feet |
Palm Cockatoo | Coconuts, nuts | 50 feet |
Bearded Vulture | Bones, turtle shells | 165 feet |
Egyptian Vulture | Ostrich eggs | 100 feet |
Conclusion
In summary, food-dropping or drop-smashing behavior has independently evolved in several unrelated bird lineages as an efficient technique to break open hard-shelled foods like nuts, eggs, bones and shellfish. Among the most prolific avian food droppers are corvids – particularly the larger raven and crow species. Extensive observational learning and practice enable young birds to master precise dropping techniques. Calculated dropping of objects from heights up to 150 feet allows birds to apply sufficient force to crack hard shells without excessive energy expenditure or damage to their beaks. By providing access to nutrient-rich foods unavailable to species lacking food-dropping abilities, this resourceful foraging behavior likely confers a selective advantage to the small subset of birds that employ it.