Crows are highly intelligent birds that exhibit complex behaviors. One interesting crow behavior that has been observed is placing rocks in water. In this article, we will explore the evidence for this crow behavior and try to understand why crows might put rocks in water.
What is the evidence that crows put rocks in water?
There are several credible reports of crows inserting rocks and other objects into water. Here are some examples:
- In Seattle, crows were observed repeatedly dropping rocks into a flood control canal. The crows would find rocks on the ground, fly over the canal, and then drop the rocks into the water below. This was observed on multiple occasions by several independent witnesses.
- At a zoo in Germany, Eurasian jackdaws (a type of crow) were seen collecting pebbles and small rocks and then dropping them into the zoo’s water features. One jackdaw was observed dropping over 30 objects into a small concrete pond.
- A man in Cornwall, England witnessed American crows dropping rocks, some quite large, into his garden pond on multiple occasions. He installed a surveillance camera which showed the crows swooping down over the pond and letting go of rocks to sink into the water.
- Beach-dwelling American crows in Canada were observed making caches of smooth stones. Researchers believe the crows likely used some of these stones as tools to crack open shellfish, while others may have been brought to the shoreline specifically to be dropped into seawater.
In addition to these anecdotal reports, a few scientific studies have also documented tool use involving rocks in crows:
- A study in New Caledonia found that New Caledonian crows sometimes use rocks to hammer open seedeaters, small mollusks found on the tidal flats. The crows will fly down to the tidal zone, gather an array of rocks of various sizes, and then bring them back to pound and crack open the shells of their prey.
- Researchers studying American crows in Maine observed the birds using rocks to raise the water level in a large tube containing a small amount of water and meat. By dropping rocks into the tube, the water level rose so that the crows could reach the floating meat reward.
While not all of these observations involve specifically dropping stones into bodies of natural water, they show crows utilizing rocks in novel ways that involve water. The reports span several crow species across North America, Europe, and the South Pacific.
Why might crows put rocks in water?
It’s not entirely clear why crows might drop rocks into water. Here are some possible explanations:
- Play behavior – Many birds and animals play, even as adults. Crows are highly intelligent, social birds that engage in various playful behaviors like sliding down snowy roofs. Dropping stones into water could simply be a form of object play.
- Using tools – Crows are well-known for their tool use. Bringing rocks to water allows them to use the rocks as simple tools to raise the water level and access floating food, as observed in the study with tubes mentioned earlier.
- Caching food – Along shorelines, crows may drop rocks into tidal pools or sea water to cache food. The submerged rocks help pin down food like shellfish which the crows can come back and retrieve later once the tide goes out.
- Marking water sources – By dropping distinctive rocks or other objects into water sources, crows may be marking these locations so they can find them again. The rock “markers” help the birds remember good places to get water or moist food.
- Hiding food from others – Crows cache food for later use but other animals sometimes steal their caches. Dropping food like clams into water and holding it down with rocks may help hide the food from potential thieves.
Several of these explanations may work together. For example, crows may both playfully drop rocks into water and also use this behavior functionally to cache or pin down food in tidal areas.
Do crows only drop rocks, or other items too?
The observations and reports focused on crows dropping rocks specifically. However, there is some evidence that crows will insert a variety of objects into water, not just rocks. For example:
- The crows inserting objects into zoo water features in Germany were reported dropping items including small sticks, acorns, pieces of bark, and even an old sock in addition to rocks and pebbles.
- Beach caches of American crows often contain a mix of items like shells, driftwood, seaweed, feathers, glass, and various human debris besides just smooth stones.
- New Caledonian crows assembling tools to hunt seedeaters will pick up and examine many kinds of objects before selecting appropriate stones to use as percussive tools.
So while rocks appear to be common items that interest crows near water, the birds are definitely opportunistic and will utilize whatever suitable objects they can find. The availability of different items likely influences what the crows end up inserting into water features.
How often do crows exhibit this behavior?
It’s difficult to say exactly how prevalent this behavior is across all crow populations. The evidence is limited to observational reports and a few scientific studies monitoring specific groups of crows. There are a few factors that suggest the behavior, while noteworthy, is likely occasional rather than ubiquitous:
- There are relatively few documented examples considering crows are widespread, visible birds that live near humans.
- In documented cases, it’s often the same individual birds engaging in the behavior repeatedly rather than all crows in a population.
- The behavior seems to occur more frequently under certain conditions like along shorelines or near abundant water sources.
In most crow communities, dropping objects into water is likely an irregular behavior exhibited by particular innovative individuals. However, in some locations like tidal coastal areas, the behavior may be more common due to the abundance of rocks, water, and potential food sources like shellfish that could reward this behavior.
Do crows ever remove items from water too?
Yes, there is evidence that crows will remove items from water in addition to inserting objects in. Some examples:
- Beach-dwelling crows will gather rocks, shells, and other items from the shoreline at low tide which are then sometimes dropped or cached at high tide.
- Studies on New Caledonian crows show they are adept at probing into water to find food and tools. This demonstrates they are willing to retrieve items from water when needed.
- In the study involving raising water levels with rocks, crows were observed removing stones from the tube after they obtained the floating food reward.
So crows do exhibit some reversal of the stone-dropping behavior, temporarily removing items from water forcaching or tool use. However, it’s not yet certain if they specifically remove the same stones they previously dropped into water sources.
How intelligent and complex is this behavior in crows?
The evidence suggests crows dropping items into water is more complex than a simple reflex. Several observations indicate advanced cognition and problem-solving skills:
- Crows select specific rocks from many choices then transport them to appropriate water sites, demonstrating intent and planning.
- They exhibit flexibility using this behavior in different contexts like food caching, accessing water, or raising water levels.
- Crows can discriminate between types of objects and choose appropriate items to drop like smooth stones vs sharp shells.
- Some crows repeat and improve the behavior over time, indicating learning, experience, and memory.
This evidence matches what we know generally about corvid intelligence. Studies show crows have tool proficiency rivaling great apes, excellent memory, insight to solve new problems, and the ability to think ahead and plan for the future. A complex behavior like dropping stones into water fits into this understanding of crows as highly intelligent birds.
Conclusion
In summary, crows are occasionally observed purposefully dropping stones and other objects into water in various situations. This intriguing behavior likely has multiple functions for crows including caching or protecting food, acquiring items from hard-to-reach places, and possibly even play. More research is still needed to uncover the full extent of this behavior in corvids and understand why they do it. But the evidence so far shows that even familiar birds like crows have the capacity to think flexibly and exhibit complex physical cognition to solve problems in their environment using tools like rocks and water.