Human/Crow Commensalism?

Some local observations and a quick survey got us thinking about a possible link between increasing bird populations and urbanization. First of all, we have noticed from being outside conducting research the last four years, the crows have seemed to increase each year. Since this was just subjective “data,” we decided to do some investigating. The results of a quick poll of walkers at Mac Anderson Park supported our idea that crow populations have been on the rise. After discussing several ideas as to why this may be (i.e. more people = more trash/food = more crows), we decided to do more digging.

When human census data from 1960-2010 is compared to crow populations from Christmas Bird Count data during that same time frame, a strong positive correlation does exist (see Figures 1 and 2). The years are placed by the data points.

Figure 1 (click to enlarge)

Figure 2 (click to enlarge)

However, this does not necessarily mean anything. Maybe crows are increasing independently from humans. Maybe there have been increasing numbers of human observers during the Christmas Bird Count events. Maybe observers have accidentally counted fish crows and ravens instead of American crows. Maybe it is all just coincidence. Then again, maybe it’s not.

The San Jose Mercury News reported in 2012 that an explosion in crow population had taken place in urban and suburban areas around the bay. The article states that crows are in the midst of a population surge and territorial expansion that has been ongoing since the 1960’s. This may be the result of one or all of the following factors: 1) mild climate; 2) lack of predators; and 3) abundant food and nesting sites that humans have provided.

John Marzluff is one of the experts trying to figure out why crows are able to live close to humans. One particular study supports the hypothesis that crow populations tend to be the highest in urban areas. This study also showed that the diet of crows was different in urban areas compared to wild areas. For example, the dominant foods eaten by crows in Seattle, WA were human refuse (prepared meats, breads, and vegetables). Crows living in a wild area along the Olympic Peninsula tended to eat equal proportions of invertebrates and human refuse.

So, besides crows, do other birds seem to follow this trend of benefiting from a rising human population, or what we would call urbanization? Barred owls may follow the same path as the crows. Field research conducted in Charlotte, NC showed that Barred owls are thriving in the city. This study indicates that there may be at least 300 pairs of Barred owls within 10 miles of downtown Charlotte.

Are there more experiments that need to be done? Of course. The questions becomes how to set up those experiments. In the meantime, watch out for the crows.


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