Journal Club: American crows: the ultimate angry birds?

Newly published research shows that crows remember the faces of humans who have threatened or harmed them, and these memories probably last for the bird’s lifetime. Not only do crows scold dangerous people, but they include family members — and even strangers — into their mob. The hostile behaviour of crows within mobs allows naïve birds to indirectly learn about a dangerous person, and to also learn to associate that individual’s face with danger and react accordingly.

Whilst in graduate school, I once lived in a Seattle neighborhood where one particular person’s presence and movements were always heralded by mobs of crows. Angry, screaming crows. In a perverse sort of way, he almost seemed to enjoy the attention, making frequent references to Alfred Hitchcock’s horror film, The Birds, whilst cawing crows swirled overhead or shouted from nearby powerlines, although he claimed complete bafflement as to the reason that hoards of birds would select him as a target for their hostility.

The crows’ reaction should probably not be a surprise to any sentient human since this man, in a testosterone-fueled rage, had shot and killed a fledgling crow a few years prior.

In contrast, the manager of the lab I worked in whilst a graduate student was well-known to the campus crows. She carried a handful of dried cat food with her each morning and dropped a few morsels at a time along her route from the parking lot to the lab. Predictably, flocks of crows followed her too, but this time, their behaviour was dramatically different: they were quieter, friendly even. They were expecting a snack.

John Marzluff is a wildlife biologist who is fascinated by crows. Dr Marzluff is a professor in the School of Forest Resources at my alma mater, the University of Washington (UW) in Seattle, where he studies corvids with an eye toward understanding how clever and adaptable they really are.

Previously published research has already shown that even though “all crows look the same” to us, individual crows recognise each other, even after being separated for many months. One of Dr Marzluff’s studies indicates that crows recognise individual human faces, too [doi:10.1016/j.anbehav.2009.12.022]. In this recently published study, Dr Marzluff continues his research with crows by finding that a crow who had a bad experience with a particular human remembers it — and crows have memories like elephants.

“Our study shows the memory lasts at least five years and counting,” said Dr Marzluff. Even though crows suffer high mortality as youngsters, those that reach adulthood can live for a long time.

“Individual crows that are adults can live 15-40 years in the wild and they probably remember important associations they have formed for much of their lives,” said Dr Marzluff.

Since adult crows are highly territorial, their knowledge of dangerous humans and other enemies should only last as long as these knowledgeable birds live and should stay in that one small location — unless nearby crows are learning about potential enemies by observing their neighbors. Such “social learning” is thought to be limited to humans and a few other social animals, although there is no good reason to not give birds credit for such abilities, too.

When individual wild birds are captured, measured and banded before release, or directly observed these activities with other crows, they identified the human handler as dangerous and reacted accordingly. Of course, this could pose a problem if you happen to be a scientist who studies wild crows.

“I felt like they recognised us,” remarked Dr Marzluff.

Over the years, he noticed that many more crows appeared to know their human foes than just those few birds with direct, individual experience (being captured and banded, or observing others being captured and banded). Dr Marzluff also noticed that the area occupied by these knowledgeable crows increased over time, extending far beyond the immediate trapping areas and including birds that could not have possibly observed first-hand the original capture events.

How did crows without any direct experience of these individual humans come to recognise their friends and enemies? Did particular individuals, who did had experience with these humans, share this knowledge with other birds? Were these crows actually communicating information with each other about particular humans? If so, how?

“Crows recruit and tolerate others of their own and different species in mobs that form around dangerous people,” said Dr Marzluff. “This social tolerance could allow naïve crows to learn about dangerous situations, locations and individual humans.”

Dr Marzluff and two colleagues, Heather Cornell, a Master’s student in his lab, and Shannon Pecoraro, an undergraduate, designed and conducted an experiment to address these questions. The objective of their study was to provide a limited number of crows with just one negative experience that they would then associate with a particular face, and then to document the subsequent behaviours of the nearby crow populations over time and space.

Over a five year period of time, the researchers used cat chow and Cheetos to lure crows close to a rocket-powered net launcher, and trapped 7-15 crows at five different locations. Captured birds were marked using a unique combination of coloured leg bands prior to release. The captured birds were held for 10-30 minutes by mask-wearing researchers, whilst nearby crows reacted to the threatening situation by forming mobs of shrieking birds that circled overhead or perched nearby. As the result of their experiences or observations, the birds in the trapping areas subsequently identified the trapping face mask as “dangerous” and reacted with hostility towards anyone wearing it (figure 1):

Figure 1. Responses of lone, unbanded crows (solid symbols, scolding; open symbols, silent) at site B. The locations of all mobs given in response to the dangerous person’s presence and the responses of all lone unbanded (never captured) crows are plotted in a spatially explicit manner for each trial. Scolding crows exposed to a mob (observed within 100 m of the location where mobbing occurred on a previous trial; these crows have a plus symbol) are evidence of peer-to-peer social learning. Scolding crows that were not exposed to a mob are evidence of social learning by observing the trapping event (trial 1), evidence of inherent scolding of a masked person (trials 2-11) or the conservative nature of our assessment (trials 2-11). [velociratorise data image]

I know the above figure is a bit complicated, but it’s really illuminating once you understand the story it tells. First, the researchers classified each location along the route as either being exposed (within 100 meters) to a mob (large “plus” sign with little black crows perched on it in the figure) or not.

To test the birds, male and female human volunteers of different ages, shapes and sizes randomly wore either the dangerous trapping mask, a neutral mask (not used for trapping) or no mask at all whilst walking for 1-2 hours along a 2-3.8 km route that included the trapping site. For each of the 11 trials, the researchers and observers — who were “blind” to the experimental objectives — observed and documented the responses of lone crows that had not been captured (unbanded crows). Those responses were categorised as follows:

• crows that scolded (black crow picture in the figure)
• crows that did not scold (white crow picture in the figure)
• crows that were exposed to mobs (“plus” sign on the crow picture in the figure)
• crows not exposed to mobs (no “plus” sign on the crow picture in the figure)

But how many crows scolded or mobbed the person wearing dangerous trapping mask, and how did that number change over time? To answer these questions, the team conducted a number of trials on their long-term study site, the UW campus (figure 2):

Figure 2. Scolding of the (a, b) dangerous and (c) neutral mask at our long-term research site at the University of Washington (UW) campus. The same responses to the dangerous face are plotted as a function of real time (a) and the number of trials during which birds could witness others scolding (exposures to dangerous mask; b). Responses were obtained during the breeding and non-breeding seasons by the authors and naive observers blind to the study design. A linear model was fit to all the data (solid lines with confidence intervals; a, c). An exponential rise to maximum model was also fit to the responses to the dangerous mask (dashed line; b). [velociraptorise data image]

The researchers found that within the first two weeks after trapping, an average of 26 percent of crows encountered scolded the person wearing the dangerous trapping mask. But over a five-year period after the trapping event, the dangerous trapping mask received an increasingly hostile response from birds in the area, suggesting that the captured birds had warned others (figure 2a). Further, the number of crows reacting to the dangerous trapping mask increased steadily over time to include naïve birds (figure 2b), whereas the number of crows scolding the neutral mask during this same time period remained unchanged (figure 2c).

Dr Marzluff and his team also found the areas occupied by scolding crows increased in size over time beyond the original trapping sites (data maps not shown here). Taken together, these data show that crows transmit their knowledge of who are their enemies — the dangerous trapping mask in this case — to their families and companions, who then pass on this information to their families and companions. This study suggests that humans who harm crows are likely to spend the rest of their lives being mobbed by increasingly large numbers of them over increasingly large areas — like the crow-killer I knew, who eventually relocated (although not because of the crows).

“Throughout most of the world these intelligent birds thrive on humankind’s waste and have increased dramatically in abundance and range,” said Dr Marzluff. But when crows invade habitats that are home to already-endangered wildlife, as is often the case these days, their cleverness and behavioural flexibility can quickly create a crisis.

Yet despite their cleverness and adaptability, corvids can also become endangered. Many corvids in the Pacific Islands and other tropical locations are endangered: one such endangered corvid, the Hawaiian Crow (or ʻAlalā), Corvus hawaiiensis, has a world population of roughly 30 birds, less than half of which survive in the wild, making it one of the rarest birds in the world.

“If you can learn who to avoid and who to seek out, that’s a lot easier than continually getting hurt,” said Dr Marzluff.

Especially when dealing with gun-toting humanoids.

“I think it allows these animals to survive with us — and take advantage of us — in a much safer, more effective way,” explained Dr Marzluff.

Crows and other corvids are very intelligent and adaptable and they are changing the public’s view of what it means to be a “bird brain”.

“[Research has shown that] some crows make and use tools, forecast future events, understand what other animals know, and — in our case — learn from individual experience as well as by observing parents and peers,” said Dr Marzluff. “These are all advanced cognitive tasks shown by only a few animals.”

Sources:

Heather N. Cornell, John M. Marzluff, & Shannon Pecoraro (2011). Social learning spreads knowledge about dangerous humans among American crows. Proceedings of the Royal Society B: Biological Sciences doi:10.1098/rspb.2011.0957

John Marzluff [emails 1, 4 & 6 July 2011]

Read more:

John M. Marzluff, Jeff Walls, Heather N. Cornell, John C. Withey and David P. Craig (March 2010). Lasting recognition of threatening people by wild American crows. Animal Behaviour 79 (3): 699-707. doi:10.1016/j.anbehav.2009.12.022

Spotlight story about John Marzluff.

In the Company of Crows and Ravens by John M. Marzluff and Tony Angell [Amazon UK; Amazon US].

Seeking the Sacred Raven: Politics and Extinction on a Hawaiian Island by Mark Jerome Walters [Amazon UK; Amazon US].

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