Birds interact with stones in a variety of ways in the wild. Some bird species use stones to aid with digestion, others use them to construct nests, and some even use stones as tools to access food sources. The question of whether birds intentionally pick up stones for specific purposes is an interesting one for ornithologists. In this article, we will explore the evidence around bird behaviors related to collecting, carrying, and otherwise utilizing stones.
Gizzard stones
Many species of birds swallow small stones and grit to aid with digestion. These stones are retained in a part of the bird’s stomach called the gizzard, where they help grind up food. Birds that eat hard seeds or grains often need gizzard stones to break down their tough food sources.
Bird species | Use of gizzard stones |
---|---|
Chickens | Eat grains, seeds, insects – require gizzard stones to digest |
Doves | Eat hard seeds – swallow stones to help crush food in gizzard |
Ducks | Eat fish, mollusks, plant food – use grit and small stones in gizzard to grind up food |
Gizzard stones are selected and swallowed intentionally by these species to serve a digestive purpose. Birds choose smaller pebbles and grit to aid this process. However, this is an example of functional rather than constructive stone use. The stones are not carried externally or used in nests.
Stone collecting by corvids
There is some evidence that corvid species like ravens, crows, and jays intentionally collect stones and other non-food objects to cache or store for later use. One theory around this behavior is that it is play or practice for food caching. Corvids are intelligent, social birds that heavily rely on stored food caches for survival. Young corvids may pick up and hide bright, interesting objects like stones in a form of play that prepares them for creating caches.
Some scientists argue that corvids, especially Eurasian jays, purposefully store caches of stones and pine cones nearby food sources to protect their edible caches from other animals. Having inedible objects mixed in with food caches may help camouflage or defend them. This suggests highly advanced planning and theory of mind by these birds.
Stone throwing by crows
The crow species is known for complex social behaviors, tool use, and problem solving skills. Some crow species have been observed to use stones as tools or weapons. For example, American crows and Northwestern crows will collect large stones and drop them onto hard surfaces to crack nuts or shellfish open so they can access the meat.
In Japan, there are reports of crows dropping stones near or at specific people they dislike or see as a threat. This seems to indicate some form of grudge-holding and intentional stone throwing, though the reasons behind it are not fully understood.
Nest building with stones
While most birds build nests out of twigs, branches, and softened mud, some species are known to construct nests out of stones, pebbles, and other rocky materials. The courtship ritual of the male African stonechat involves building several stone nests and inviting females to select one to breed in.
Female Australian flamingos will often construct a cone of mud and stones to lay a single egg on. The stones may help stabilize the nest or raise it above wet ground. Laysan albatross also use pebbles and debris to create a stony nest mound before the female lays the egg on it.
For these birds, stones are intentionally gathered and put to use in nest construction. The stony materials are chosen for their structural qualities rather than decoration.
Do some birds play with stones?
The purpose behind certain stone-related bird behaviors, like corvid stone caching and crow stone dropping, remains somewhat mysterious. It is possible that play, curiosity, and intelligence contribute alongside more practical factors.
Young birds especially seem prone to playing with stones the way they would play with toys. Magpies have been observed juggling stones, carrying them around, and even seeming to show them off to other nearby magpies. There are accounts of other corvids playing games of toss with rocks.
Ravens are also known to play with stones by dropping them in water and watching the splashes. Some birds even seem to develop preferences for specific stones and become attached to a “favorite” one.
It’s likely that play involving stones and other objects contributes to cognitive development in birds. Young birds that manipulate objects and solve puzzles may gain physical skills and strategic thinking ability. Some element of novelty, curiosity, and fun probably occurs alongside more functional reasons for birds interacting with stones.
Tool use involving stones by birds
Using stones as tools to achieve a goal requires advanced intelligence and cognition. Corvids like crows using stones to crack open food sources is one example. Some other interesting stone tool use behaviors in birds include:
Andean mountain cavy
This South American bird swallows small stones and regurgitates them to use as weapons against predators or threats. Regurgitating the stones takes skill and forward planning.
Egyptian vultures
When breaking open large eggs is difficult, these vultures have been observed carrying stones in their beaks and throwing them forcefully at the eggs from heights until the shell cracks. This demonstrates tool use and gravity comprehension.
Bearded capuchin
These small monkeys crack palm nuts by placing them on flat stone anvils and striking them with smaller handheld stones used as hammers.
Sea otters
Sea otters are renowned for using stones as “anvil” surfaces to crack open shellfish against. This requires identifying appropriate shaped stones during foraging and dwarfs.
Elephants
There are reports of elephants using kicks or hits from stones held in their trunks to drive away threats or annoying birds. The stone is used as a projectile weapon requiring some calculation.
Why do birds intentionally handle stones?
Based on available research, these appear to be some of the reasons certain bird species intentionally pick up, carry, or utilize stones:
Digestion
Birds like chickens and doves swallow small grit and stones to accumulate in their gizzards and assist with grinding up food.
Nest building
Some flamingos and albatrosses build nest mounds from stones, probably for stability and structure.
Caching strategy
Intelligent corvids may cache inedible objects like stones near food sources to protect caches from theft.
Food access
Species like crows use stones to crack open tough foods like nuts or shellfish otherwise inaccessible to them.
Weapons
Andean mountain cavies regurgitate swallowed stones to throw at predators. Elephants may wield stones in their trunks to drive off threats.
Play
Young birds often playfully interact with stones in ways that may prepare them for adult tool use.
Problem solving
Some birds appear to use stones to solve challenges like breaking open eggs (Egyptian vultures) or accessing food puzzles.
Curiosity
Highly intelligent species like crows and ravens often manipulate stones in ways suggestive of curiosity and interest.
Conclusion
Many birds have functional reasons for interacting with stones related to digestion, food access, construction, and defense. Additionally, some birds appear to play with and explore stones out of curiosity or boredom. Highly intelligent corvid species and tool-using birds also purposefully select stones to solve problems and challenges in innovative ways. This suggests advanced cognition around stones as tools.
Overall, the evidence shows some birds do intentionally seek out and handle stones for a variety of reasons. However, most interaction with stones by birds can be explained by fairly practical evolutionary purposes, with only the most intelligent avian species likely collecting stones purely out of curiosity or amusement. Understanding why different bird species use stones helps shed light on their intelligence, ecology, and behaviors.