Woa, look at all those crazy crows! Why are they swarming like that?
One of the most striking displays of crow behavior is when crows gather together in massive groups at the end of the day.
These flocks can grow to as many as hundreds or even thousands of crows, which really makes them stand out in the environment.
So if you’ve ever seen this massive influx of noisy black corvids, you might have wondered… Why do crows gather like this?
Crows are communal sleepers, so they gather in massive groups to roost together at night.
As the sun begins to set, crows will fly in from long distances to a central location where they can share warmth, safety from predators, and even exchange information relevant for survival.
Wave after wave of crows will swarm together in tree tops, on the roofs of buildings, or on the ground until there are thousands of crows all gathered in one place, before finally moving into their nightly roost location to sleep.
Bird watchers have marveled at these massive gatherings of crows at dawn & dusk for hundreds of years and speculated about why they form such large groups.
There are actually multiple reasons why crows do this, and researchers are still working to piece together the complete story. Here’s what we know so far…
Safety In Numbers
The simplest and most common explanation for why crows get together in such massive groups is for safety from predators.
Crows do have lots of dangerous predators like owls, hawks & eagles.
During the daytime, crows are extremely skillful at scolding and chasing these dangers away (A behavior we’ve discussed in another article on crow language).
This is why when you see groups of crows all suddenly start cawing & going crazy during the daytime, it’s very often because they’re reacting to a predator.
However as nighttime approaches, crows lose their ability to see in the dark which leaves them more vulnerable to nocturnal predators like owls.
Sleeping in large groups means there are more crows to help out and provide flock safety if something happens at night.
It also helps to explain why crows choose sleeping locations in urban environments, as the artificial streetlight likely provides further protection by helping them see danger more easily.
Shelter & Warmth
Another important reason for crows sleeping as a group is the ability to share shelter & warmth.
Crows will typically choose communal sleeping locations in sheltered urban environments that provide extra warmth & protection from the elements.
They gain a few extra degrees of warmth and wind protection by commuting into these sheltered sleeping locations at night, making it that much easier to conserve energy through the long winter.
This makes a lot of sense when you consider that communal sleeping groups are mainly used from fall to early spring, during the coldest and darkest time of year.
So shelter & warmth is almost certainly part of the motivation for group roosting crows.
However there’s one other reason that’s possibly the most fascinating because it really shows the incredible intelligence of crows.
Exchanging Information About Food
A fascinating discovery about evening crow gatherings is crows appear to be exchanging information to help the group find abundant food sources the following day.
This is important because food is generally much more scarce for crows during winter than it is during summer.
There are no insects, worms or cold-blooded creatures for crows to find on the ground when everything is frozen and dormant.
However there are still plenty of high value foods for crows in a winter landscape if they can simply find the right opportunities.
Something like a dead deer or moose is very well preserved in the cold temperatures and provides huge amounts of food, but these high calorie opportunities are also much harder to find.
So if there’s an animal carcass lying in a forest somewhere, the crows still need to discover it without wasting too much energy on the search.
This is where these evening roosts serve as exchange hubs for information.
Research done on crow roosting behavior seems to indicate that crows are actually communicating the location of these bulky food sources during their nightly gathering rituals.
This is supported by examining pellets of undigested food below the roost and observing that the number of crows feeding on the same carcass increases with each passing day.
On day 1 you might find just one or two crows with pellets containing material from a food source.
Then on day 2, you find evidence that more crows are feeding at this location.
Day 3 is even more, and so on until the food source runs out.
Amazingly, we can track that the word of these locations spreads out through crow populations who sat close together in staging areas on the previous evening.
The idea is while crows are making all their crazy displays and calls and body language at their nightly gathering, there’s some sort of meaningful communication happening about food.
How exactly they communicate the specifics is unknown, but it is pretty darn fascinating and just goes to show there’s a lot more to crow intelligence than previous realized!
As a side note, this information exchange has also been observed in raven populations who also roost together, though in smaller numbers. (What’s the difference between crows and ravens?)
Other Possible Crow Gathering Scenarios
Aside from their nightly roosting behaviors, there are a few other reasons why crows gather in large, noisy groups.
This is especially true if the gathering of crows happens during late spring, summer, or anytime before the real cold of autumn starts to arrive.
Predator Response & Territorial Behavior
Probably the most common reason crows group up and make all kinds of noise is because there is some kind of predator in the area like an owl, hawk, or eagle.
This is surprisingly common, and even a group of just 5-10 crows gathering in a tree making loud calls can be pretty darn captivating.
Nearly identical gatherings can also occur as territorial behavior during the breeding season when a family of crows is chasing away other crows or ravens.
Crow territorial & alarm behaviors are almost identical, but the cause is usually fairly easy to discern.
Crows will also gather in pretty big groups when there’s some kind of high energy food source that can be easily stolen like fish or meat.
Crows will team up to steal food from eagles, hawks & ospreys, and their displays of crazy cawing & chasing around the sky are quite common to see.
However these food stealing groups typically involve groups of only 5-30 crows or so, which is a far cry from the hundreds and thousands of crows that gather for nightly roosting.
Food stealing can happen in any season, and at any time of day so it’s important to recognize this behavior if you want to confidently sort out what the crows are really doing.
Another behavior that has gained a lot of attention is crow funerals.
This is a pretty fascinating activity, and it gets a lot of press because it resonates with our human emotions and shows the incredible intelligence of crows.
Crow funerals are closely linked to predator evasion and defence strategies, as we discussed above.
So if you want to learn about crow funerals, the best place to start is by observing their predator evasion & mobbing responses.
There’s evidence to suggest the purpose of crow funerals is to help crows teach each other about possible dangers in the area.
This means that after a crow funeral, it’s extremely likely that you will observe increased vigilance & alarm activity in the area.
An important pillar of crow communication is to realize that crows react in predictable ways to the presence of dangers like hawks, eagles, owls, cats, coyotes, and even humans.
I’ve done lots of study on how these communications take place so if you’d like to learn more, check out my free book – What’s That Crow Saying?
Crow Roost VS Predator Evasion VS Territorial Behavior
So with all these different reasons for crow gatherings, how do you know what’s really happening out there?
The easy way to determine why crows are gathering is to answer the following three questions:
1. How Many Crows Are Involved?
If it’s less than 20 crows, most likely you’re seeing some sort of response to predator, food stealing or territorial behavior.
If it’s more than 50, there’s definitely a good chance that this could be related to communal roosting, but you also have to consider the time of day.
One of the hallmarks of crow behavior related to roosting is that it often involves hundreds or even thousands of crows.
2. What Time Of Day Is It?
If you see crows swarming together in the middle of the day, this should be a clear sign that the gathering is not related to their sleeping habits.
If however, the crow activity is happening near the start or end of the day, this very likely is preparation going to or from the roost.
Bear in mind that when the days are short, sometimes this daily commute of crows to their central roost starts in the late afternoon so they can arrive before it gets dark.
The third factor to consider here is season.
3. What Season Are You In?
It’s important to remember that crow group roosts are primarily active during the non-breeding season, during the coldest and darkest times of year.
This means from mid-spring, through summer and early fall, there’s much less roost activity compared to fall through winter and early spring.
The 3 Stages of Crow Roosting Behavior
There are 3 stages of behavior that crows go through when preparing to roost in big groups.
Here’s what to look for:
The Daily Commute
First – look for big groups of crows flying high in the sky towards a single direction, most likely moving towards a city.
Depending on how far you are from the actual roost, these crows will be flying as much as 5 or 10 times the canopy height, which is significantly higher than you typically see crows in the sky during normal daily activities.
This usually happens at the stage when the sun makes altocumulus clouds turn a bright red color (learn more about different types of clouds).
They will come in staggering waves of dozens and eventually hundreds of crows flying nonstop overhead for many minutes.
You’ll notice they’re all moving in the same direction, and it will happen repeatedly around the same time every day.
At this point you might still be several kilometres or miles from the actual roost location, but as they get closer, the crows will move lower in the sky as they prepare for staging.
The Staging Phase
If you follow this group of crows through the sky for long enough, eventually you’ll see them dropping altitude to a more normal height in the sky.
At this point, you will most likely be in an urban or semi-urban environment, much closer to the actual roost location.
Here the crows will gather in treetops, on the roofs of buildings, or spread out on the ground.
It’s not uncommon to witness massive waves of hundreds or even thousands of crows gathering in these staging locations, which may vary through different locations from day-to-day.
The Roost Phase
Keep watching these staging areas, and after a few minutes the crows will fly off once more in massive waves of activity.
At this point the day is truly coming to a close, so crows are moving into their final roost.
Crows will typically roost in the same location for many years unless something changes in the environment.
The exact locations of these roosts are often well known by local people in the area because it’s hard to miss thousands of crows flying to the same spot every day!
The Next Morning
At dawn, the whole process repeats in reverse. Starting first thing in the morning you’ll see crows moving out from the roost in waves.
Further out you’ll see them flying high overhead as they head out to start their day of foraging and causing their usual crow mayhem.
So there you have the fascinating scoop about what crows are really doing when they gather together at various times, seasons, and group sizes.
Now it’s your turn to get outside and go watch some crows!
Terry Woram says
Thanks for the insights! Love crows, ravens and our backyard scrub jays – Corbin’s are the best! We are watching hundreds of crows stream flying over SFO airport to their evening roost. Amazing, and so determined! I’ll look up your language resources.