The Growth of Crows in Cities

| |

Urban expansion has spelled doom for many animal species. The development comes hand-in-hand with fragmentation and destruction of natural habitats, leaving many populations without a suitable habitat, food, or a pathway to communicate, mingle, and breed.

Well, not the case with the American crow, though.

In 2012, BirdLife International estimated the American crow’s (Corvus brachyrhynchos) population to be around 31 million, propelling them into the top 5 most common US bird species. However, what makes them special is not just their sheer number, but also the fact that they have an established presence in urban areas, successfully breeding and abundantly roosting in towns and cities.

The phenomenon is not new. According to the Humane Society, crows have been abandoning their rural winter roosts and opting for cities and towns instead ever since the 1960s.

Avian Einsteins

It’s not just the US, and it’s not just the one species of crows – corvid populations worldwide have responded to urbanization by thriving and increasing their population. The “Avian Einsteins,” as the intelligent family of birds sometimes gets called, have a knack for the cities, and we are still exploring why exactly.

Why are There So Many Crows in Cities?

Here is what we know about the factors that have driven the rise of American crow population.

Food is abundant in the city

When it comes to food, crows and corvids are the opposite of specialists. Besides their natural choices, such as invertebrates, small vertebrates, carrion, berries, and nuts, crows will gladly gobble up any human food that comes their way.

A crow stand on a bird bath.
A crow using a backyard bird bath to soften some food. Photo: Caitlin Dempsey.

Digging through trash is one of the crow’s specialties. Their cleverness allows them to use tools and excel at problem-solving, so it’s no wonder that manipulating the discarded food packages and reaching any food remaining inside is an easy task for them.

Since the average US household wastes nearly 40 percent of its food supply, crows are thriving on this resource. They will often pick up food at landfills, in dumpsters, and in trash cans.

Family feeding time

The way crows feed their young also influences their success. Crows are luckier than songbirds that rely on insects and other invertebrates to ensure their offspring’s early survival. Like pigeons, crow parents regurgitate pre-digested food for them, although only for the first few days of their lives. 

While it may not sound like a jackpot to squeamish humans, the balanced “bird milk” allows for the extraction of nutrients from any food the parents could find, overriding scarcity of particular food items. After the regurgitation phase, the parents start bringing other food items that are common in the cities – maggots and earthworms – and after these move onto larger, meatier meals.

A group of crows looking for food in the grass in a suburban area.
A group of crows looking for food in the grass in a suburban area. Photo: Caitlin Dempsey.

Also, crow parents are not alone in the efforts to feed the youngest members of their families. Like with close-knit human families, older offspring from the previous generation stays with parents for up to 5 years, and in that time, readily helps feed their younger siblings. 

As crows do not breed until they are at least two years old and raise only about three chicks per nest, this strategy has proven highly successful. The complex family life seems to be an advantage for surviving and thriving in the human-dominated realm.

Urban roosting areas for crows

Since crows form large flocks – the so-called murders of crows – when roosting, naturally, they need a large shelter, even though their need for individual space is negligible. Unfortunately, in rural and other non-urban areas, the land is cleared for agriculture and other purposes, and large trees – their natural first choice for roosting – are often logged.

However, strands of street trees and park trees in the cities are usually protected; also, large, tall buildings are a perfect anthropogenic crow roosting resource. In general, towns and cities have become more roosting-friendly than the average modern countryside.

Crows could be safer in cities – and know it

Crows have been seen as a countryside nuisance for a long time. Due to their frequent foraging of crops and cases of attacks on newborn livestock – especially lambs – crows have had a bad reputation for centuries and were frequently persecuted and killed. Hunting non-migratory birds damaging to agriculture is still legal in some states. However, in the cities, there is no agriculture and no imminent reasons for the mistreatment of crows.

Mounting evidence shows that crows have parent-to-offspring and peer-to-peer knowledge transfer about threats. In simpler terms, crows can communicate and teach each other about who their enemy is and presumably where the danger lies. In time, the information spreads among a larger population of crows than the one that initially experienced the trauma and over a greater geographic area.

A group of researchers conducted an experiment – they trapped, banded, and released a certain number of crows in five sites in Seattle while wearing a “dangerous mask.” At first, only the crows that witnessed the events knew that the mask represented danger and scolded (alerted) each time they would see the person with a mask passing by. 

In time, the “dangerous mask” knowledge has spread horizontally – among peer crows and horizontally – from parents to offspring. By the end of the five-year research, scolding behavior had doubled in frequency and spread at least 1.2 km from the place of the initial trapping.

In the same light, it is suspected that crows might have been to gain and spread the knowledge that urban areas represent a safer, plus more abundant habitat for their species.

Other reasons crows love cities

There are indications that crows also enjoy cities because of their higher average temperatures – a phenomenon called the urban heat island.

Also, artificial lighting might make them feel more secure because it helps them see night predators such as owls.

The Main Observations on City Crows

In 2001, John M. Marzluff and his colleagues did one of the pivotal studies on the American crow’s urbanization response. They did this by comparing the rates of winter population change in urban and non-urban locations and pooling studies from New York, Wisconsin, Washington, and California to derive conclusions about crow’s urban space use, reproduction, and survivorship across the US territory.

The study confirmed that the crow population tends to be the densest – and to increase – in North American urban areas. They concluded that this was at least partially owed to the modest space needs of crows in urban areas. The survival rate was also high across the urban environments; however, reproduction and population growth peaked in suburban and rural areas. 

Crows roosting in a tree at dusk.
A flock of crows roosting in a tree at dusk. Photo: Caitlin Dempsey.

On the basis of these findings, the team hypothesized that the urban crow population actually got its boost from surplus crows from non-urban areas rather than being exclusively bred in urban areas. 

The phenomenon is likely fueled by crow’s social nature and may be especially true in the western US, where pre-breeding crows form flocks in an effort to gain access to abundant urban food sources. However, in midwestern and eastern areas, where crows migrate for winter or remain with parents to help rather than roam as pre-breeders, the young crow dispersal cannot adequately fuel urban population growth. 

Conclusion

One thing is sure. Crows and humans have entered a strange and uncalled-for symbiotic relationship that is here to stay. While often seen as a nuisance, crows are here only because they have created a perfect habitat for a social, height-loving, highly intelligent, scavenging omnivore. 

Even when they pick through your garbage and make a mess – remember that the mess and the waste actually come from us, and the Einstein corvid cleanup crew is just trying its best to fix it.

Resources

The studies

Cornell, H., Marzluff, J., & Pecoraro, S. (2011). Social learning spreads knowledge about dangerous humans among American crows. Proceedings. Biological sciences / The Royal Society. 279. 499-508. 10.1098/rspb.2011.0957.  https://doi.org/10.1098/rspb.2011.0957

Marzluff, J., et al. (2001). Causes and consequences of expanding American Crow populations. Avian Ecology and Conservation in an Urbanizing World pp 331-363. https://doi.org/10.1007/978-1-4615-1531-9_16

Articles

Danville home to North America’s largest crow roost. The News-Gazette. 4 January 2004. https://www.news-gazette.com/news/danville-home-to-north-americas-largest-crow-roost/article_a549b72c-9039-5a01-a652-e217e4de8703.html

McGowan, K.J., Family Lives of the Uncommon American Crow. Cornell Plantations Magazine, vol. 51 (1): 1-4, Spring/Summer 1996. https://www.birds.cornell.edu/crows/planta.htm

Urban Crows Fact Sheet. HSUS. https://www.humanesociety.org/sites/default/files/archive/assets/pdfs/Urban-Crow-Fact-Sheet.pdf

What to do about crows? HSUS. https://www.humanesociety.org/resources/what-do-about-crows

Related

Share:


Enter your email to receive the Geography Realm newsletter: