Researchers at the University of Washington have found a species of crow that distinctly alters its behavior when attempting to steal food from another crow, depending on whether or not the other bird is a relative.
The Northwestern crow (Corvus caurinus) uses a passive strategy when it attempts to take food from kin but becomes aggressive when it tries to steal a morsel from a non-related crow. This is believed to be the first time that such a behavior pattern has been observed in any bird species.
The findings are published in the current issue of the journal Bird Behavior by Renee Robinette Ha, a UW lecturer in psychology, and James Ha, a research associate professor of psychology. In a companion paper to be published in the next issue of the journal Animal Behavior, the Has, a husband-wife team, quantified scrounging or thievery attempts among Northwest crows. When birds found valuable items such as small fish or clams, other birds tried to steal the food 46 percent of the time and 41 percent of those attempts were successful.
"This research shows these birds discriminate kin from non-kin," said Renee Ha. "They can tell who they are related to and treat birds differently. We know it is more complex and sophisticated than being based on just the birds they know. Crows and other corvids (ravens, jays and magpies) are highly complex cognitively and socially, and are very adaptive."
Earlier work by the Has showed that thievery is common among these birds, which are constantly looking for an opportunity to filch a snack from another crow.
To understand crow behavior, the UW researchers captured and banded 55 birds that foraged in a suburban Snohomish County park along Puget Sound north of Seattle. They also drew a small blood sample from each bird. The bands enabled the researchers to identify individual crows. DNA analysis of the blood allowed the Has to determine which of the banded birds were related.
The researchers observed the crows for 223 hours over the course of 2½ years, looking for instances of thievery involving two banded birds. The crows' behavior was remarkably different, depending on the target of a theft.
When the birds are related a crow will use a passive strategy and "walk up to or kind of sidle next to the bird with the food. Often the second bird will give up the food to the scrounger," said Renee Ha. "With aggressive scrounging, there is usually a flying approach by the thief who nearly lands on the other bird. This can be followed by vocalization, physical contact and attempts to take the food. Usually it also involves chasing and avoidance by the bird with the food."
She added that there does not appear to be any other pattern associated with scrounging. The sex or age of the birds do not seem to be factors and the birds will steal from relatives as well as non-relatives.
The researchers noted that the majority of the crows they observed engaged in hunting for food, as well as thievery. A few crows only hunted and none exclusively relied on stealing.
"Crows can not steal for a living because there are not enough opportunities to steal enough big food items. Scrounging fills a gap and crows will do it if the bird next to them has high value food that takes time to swallow," Renee Ha said.
James Ha added that the Northwest crows have complex social groups. "They tend to associate in stable groups that contain some of the same birds all of the time. But these groups are not kinship groups and the members are not all necessarily related to each other. They will try to steal food from familiar birds, kin or strangers," he said.
"This behavior seems to be specific to Northwest crows which are shoreline feeders of large prey, said Renee Ha. "Most people, regardless of where they live, are familiar with the more common American crow (Corvus brachyrhynchos), which feeds on worms and grain. That kind of food does not offer targets for thievery."
Co-authors of the study were Paul Bentzen, a former UW associate professor of aquatic and fishery sciences who is now with the biology department at Dalhousie University in Canada, and Jennifer Marsh, a UW psychology doctoral student. The research was funded by the Animal Behavior Society; Sigma Xi, a scientific honor society; and the UW.