Halloween Edition

October 2014

Research Recap October 30, 2014

A Murder in the City

Alfred Hitchcock’s movie “The Birds” dramatized the occasionally spooky nature of birds. If you’ve seen the movie, you may get an uneasy feeling whenever you see large numbers of birds roosting together on a dark night. Today, seeing thousands of crows, or a “murder of crows,” roosting in a downtown park on a chilly fall evening is not an uncommon sight.

But why are crow populations so large—and growing—in urban areas? That’s just what Dr. John Marzluff of the University of Washington set about to understand along with Cornell Lab’s resident crow expert Kevin McGowan and colleagues in their study, “Causes and consequences of expanding American Crow Populations” which appeared in Avian ecology and conservation in an urbanizing world.

Celebrate Urban BirdsPhoto © Kevin McGowanAmerican Crows are a focal species of the Celebrate Urban Birds project. Learn more about American Crows

About Crows

Crows are synanthropic—they live near, and benefit from, humans and the artificial habitats we create. In fact, crows are rarely found breeding greater than 5 kilometers from human activity.

Besides having an affinity for landscapes modified by people, crows have a social nature much like ours. Unlike most other birds, young crows stay with their parents for years helping them guard their territories and raise their siblings. Crows are also gregarious and intelligent. They have been known to make tools to get food, trick dogs to get access to their food bowl, and distract otters to steal their fish.

The Importance of Food

In their study, Marzluff and colleagues compared the diets of crows living in urban and suburban areas vs. the diets of crows living in wildland (areas not modified by humans) and exurban (communities beyond the suburbs) areas. Do you notice anything interesting about what wildland crows eat compared to their urban counterparts?

Diets Graph

They also looked at the foraging areas of crows within six study areas (along a gradient from urban to wildland). The graph below shows how much more area is needed for a wildland crow to find enough food each day than an urban crow. Living alongside humans appears to make food much easier to find.

How Far Do Breeding Crows Travel For Food?

Foraging distances of breeding crows in acres

1. Enter your address     2. Select a circle to visualize how far that really is based on a place you know!

Relative Foraging Area

Urban
Suburban
Rural/
Exurban
Wildlands

Keeping up the population

Finally, the researchers turned their attention to survival and reproduction rates in the four habitat categories (urban, suburban, rural, and wildland) to explore patterns that could account for the growth of crow populations in urban areas.

Survivorship was found to be high across all study areas, while breeding success varied among the study areas.

Survivorship & Breeding Success Plot

These data allowed the team to create projections of population growth based on reproductive success and survivorship in urban and wild areas. Notice where the crows had the highest level of survivorship.

Projected Crow Populations By Study Area

Starting at 50 crows per population

As you can see from the graph above (which is only a very rough model used for comparisons and not intended as a realistic estimate of crow population increases), crow survivorship remains high (the upward trend line indicates this) in all habitats, but the projected population growth based on reproductive success does not account for the growth the of crow populations in urban areas. So, what is going on?

Conclusion

This led the researchers to hypothesize that urban crow populations are increasing primarily because surplus crows are leaving suburban and rural areas and moving into the urban areas where food is more abundant. In nature, though, nothing is so cut and dry: after studying pre-breeding flocks of crows, the researchers found that their hypothesis may only hold true in the west, but not in the east or midwest.

This is, perhaps, best captured by the words of the researchers themselves:

In the western United States, where pre-breeders often form flocks able to exploit urban riches, our dispersal hypothesis may be accurate. But, in midwestern and eastern areas, where crows migrate south for winter or remain on territories to help rather than float as pre-breeders, dispersal may not be adequate to fuel urban population growth.

Crow populations in urban areas are increasing across the United States, but we don’t know why city crow populations are increasing in the east and midwest. Why do you think it’s so? Let us know in the comments below. And don’t be surprised the next time you find yourself in a city and you witness a murder of crows.

To read the full research paper, visit http://www.birds.cornell.edu/crows/Marzluff%20et%20al%202001%20Avian%20Urb%20Ecol.pdf.

Counting Crows

Help track crow populations by joining Celebrate Urban Birds, Project FeederWatch, or eBird. And if you want to learn more about crows in urban settings and help track urban birds, visit Celebrate Urban Birds.

This Month’s Articles

A Murder in the City

Research Recap

All Those Pumpkins!!

Factoid October 23, 2014

Myths of the Ghost Bird

Interactive October 16, 2014

One of the Flock

From Our Data October 9, 2014

Along Came a Spider

Slideshow October 2, 2014

16 Comments

  1. Christine Meadows says:

    Crows in rural areas depend on insects as a major food item. In the Midwest where I live, the insect population is probably being slowly eliminated because of the use of neonicotinoid based pesticides on corn and soybean crops. Hard to believe but we know what has happened to the Monarch butterfly and also bee colony collapse disorder because of the use of systemic pesticides. Eventually, birds that depend on insects will be affected too.

  2. Fig Simpson says:

    I suspect Crows are attracted to the warmth. Sleeping on sunwarmed rooftops of heated buildings in a concrete environment is penthouse living. Plus their play time is extended by the all the lights. They can be a bit of a night owl without worrying about actual night Owls which roost in trees. TV antennae make fantastic boardrooms for company meetings in the mornings and late afternoon. And generally the height of buildings is attractive. I think it would be very interesting to compare Crow populations with areas’ average building heights. Looking at Japan, many buildings are around 10 stories high, with a short wall serving as a nice windbreak, and warm black tar surface. I suspect the dry air over a building is at least a couple degrees warmer than in a damp wooded area. Changing rooftop designs would discourage nesting, roosting and population explosion. Here in Japan, being a landlord is a dream job; they never do anything, nevermind checking their rooftops.

  3. Diane Cann says:

    Not being very scientific here, but I think that crows are increasing due to their ability to train people to feed them. I live in the southern tier of NY and had a family of crows, 5 to 7, in number who came to my house every morning at 7:00 to be fed. Actually one crow would arrive first and caw, caw, caw. Then I would see two more arrive as I cooked them scrambled eggs in the microwave. I would chop up the scrambled eggs and by the time I got outside a couple more crows would be swooping overhead. Once back inside I would watch them from the window as they gobbled down a piece or two and then hop around the backyard with pieces of egg in their beaks to bury for later. They came every day for about 10 years and then mysteriously disappeared for a year. I think they got sick of eggs. Now I have a new family starting the same routine, with eggs. They have me very well trained. Urban areas have a lot more people craving nature and one way to get your nature fix is to feed birds.

  4. Julie Aitchison says:

    Interesting to read that someone else has had a family of crows for so long. We have had a continuing crow family for 33 years now in a semi rural/suburban area (across from a small lake) in the lower Hudson Valley in NY. We’ve been avid birders (amateur and professional) for years and I now keep count on ebird, but wasn’t a part of it until a year ago because of some circumstances. Our crow family introduces their yearly offspring to us in June. They call at our house just after daybreak. They “talk” and cackle to us, and scold us if we don’t feed them. We give them leftovers, fat from meat, raw chicken insides, and anything else we have too much of. Our daughter even did an actual 6 mo. scientific study on how they are protein selective when offered different kinds of foods at the same time. Anything meat/fat related gets eaten/taken first. Starchy stuff like pasta, bread, mashed potatoes (which they lap up with their tongues) is next, veggies last, but they absolutely hate peas, and stomp on them time and time again. We have even witnessed their own kind of “funeral” with grieving the death of one of their members. We love our crows!!!! Life in this household would never be the same without them!

    • DEAN ALLEN JONES says:

      I wanted to get a description of the “funeral rites” behavior that your crows exhibited when one of them died. I’m not sure I’ve ever seen or heard of such behavior before.

  5. Barbara Resheske says:

    My crows have a sentinel bird that watches as I let my dogs out first thing in the morning, he calls his peeps as soon as I toss out hard dog food, or whatever I have on hand.Soon after I dig a new space in the compost pile and drop in the veggie peels, they’re right there pecking around for some goodies. When I worked at the local library 1/2 block away, they followed me there and called from the trees! With students, I made recycled peanut butter/seed toilet paper feeders,hung them in the trees in the library yard, and did the Christmas bird count.As long as I kept the crows well fed at my house across the street, the crows politely left the peanut feeders alone…That was very unexpected.
    Last year they had a chick who cried and cried a sad tune, I believe it was ill.So, I brought food to that yard for him/her.Eventually the crying stopped, BUT, several crows then came to my yard and imitated that mournful cry to obtain more and better food! Incredible! I look forward to my flock who aren’t even afraid of my little dogs who are about the same size as them.They strut around my yard as if they own the place…too funny!

  6. Kate says:

    These stories about crows in people’s yards reminds me of an interesting book I read recently called “Crow Planet” available in e-book as well as paperback. Lots of interesting information about crows as well as musings on our place in the scheme of things.

  7. Ferris says:

    I was fortunate enough this past Saturday to encounter a large group of American Crows roosting in trees along Tower Road on the Cornell University campus, in Ithaca, New York. It was very dark at the time, but there was enough light reaching the clouds in the background to see them. Check out this video, recorded during a live stream to Ustream, which was enhanced to better show them. The best part begins at about the 8:45 mark, with many of them briefly taking to the air shortly afterward. http://youtu.be/UiFBFkItW1Y

  8. Thank you for sharing excellent information. Your website is very cool. I am impressed by the details that you have on this blog. It reveals how nicely you understand this subject.

  9. Ross Dobson says:

    Cornwall, Ontario, Canada: Not sure if this is relevant to above paper, as the issue of winter crow roosts in urban areas is different than the above discussion. In our local case, the winter (seasonal) use of our urban space (city) is a nighttime use primarily, with adjacent rural crows flying into the city each evening and flying out each morning, presuming to their dispersed (rural) feeding areas . Thousands of crows do this each night here as they do in other urban zones, and the other observation which seems to be unusual or unique to our winter “roost” is the behaviour of many of the crows remaining on the ground overnight, even when there are adjacent trees avaialble. The main factor in this behaviour seems to be the degree of ambient light sources for the crows which they prefer, probably as a risk reduction factor to help watch for owls. Photos are available of this ground roosting behaviour here.

  10. Felt great reading this post, at least there are people who think about these living creatures which makes our environment look so natural and beautiful.

  11. Nice Post, Glad I stumbled Upon this post.

  12. Meret Wilson says:

    Interesting article. Have noticed a huge increase of Fish Crows in FL. Both Fish and American are here ever more mixing. Would like to see an article re their calls. In winter both caw, not hearing the identifiable Eh Eh nasal sound of Fish Crow so readily. Help in this, especially for accurate CBC counts would be greatly appreciated.

  13. It is excellent information, keep it

Post A Comment

Your email address will not be published. Required fields are marked *


Unfortunately, this website does not support your browser as it relies on modern technologies. Please update your web browser. It is free. Please consider using a modern web browser like Google Chrome or Mozilla Firefox for a better web experience.