Too Easy for Crows
The urban
ecosystem is one that we often forget to consider when we think of ecology. It
is much easier for our minds to associate nature with views of vast green
fields or lush forests full of wildlife, but, for me, it is an awesome change
of pace to see the same complex organization of trophic levels right in the
place where I live. Due to humans continuing to diminish natural habitat, more
and more animals and plants have to make tough decisions about their survival. Recently,
my lab partner and I started an experiment on raccoon foraging in urban areas
that showed evidence of these kinds of decisions. The next day we found that
crows had eaten all of the eggs we used as raccoon bait! Since we do not live in
an area that is by any means overpopulated with crows, this event turned my
attention to what caused the crows to forage so heavily in our research area.
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| Being in an open space forces crows to be more vigilant; thus, forage less. However, it is much easier to spot resources in open spaces, which makes foraging more efficient. Retrieved from http://texascrowpatrol.com/Regulations.html |
A
study by Ward and Low in 1997 examines the impact of different aspects of the
urban environment on crow foraging based on crow vigilance (http://www.jstor.org/stable/4163843?seq=1).
They observed and measured the vigilance of the crows in an urban college town,
much like ours. Since vigilant behavior and foraging are mutually exclusive,
observing the amount of time a crow spends scanning its environment (being
vigilant), tells a lot about its foraging patterns. The authors found that the
crows became more vigilant as human disturbance increased; thus, foraging
decreased. In addition, many factors such as group size, time of day, and
duration of the day’s precipitation showed an effect on crow vigilance and
foraging. However, the effect most applicable to our situation was that
decreases in the distance from cover protective cover caused for greater
foraging. This could explain why the
crows would be particularly comfortable foraging in our study site that has
plenty of tree cover. |
| Eggshells are all that remain after the crow's visit. It was just too easy. |
In fact, our study site is better
than just having good tree cover: it has very few shrubs creating almost no
visual obstruction between the eggs on the ground and any bird flying under the
tree tops. Further, the property is located just off of
one of the least busy streets in our small town. Though we are attempting to
use this street for a treatment of human traffic, we have created an almost perfect foraging environment for the crows: easy targets with low risk
required. It is no wonder they got to the eggs before the nocturnal mammals
did. So to all of you reading this and trying to decide where to lay your eggs,
I would suggest finding a spot where the crows need to be a little more
vigilant. I’m sure even the crows would appreciate a challenge.
Reference:
Ward, C. and B.S. Low. 1997. Predictors of vigilance for American Crows foraging in an urban environment. The Wilson Bulletin 109: 481-489.
Reference:
Ward, C. and B.S. Low. 1997. Predictors of vigilance for American Crows foraging in an urban environment. The Wilson Bulletin 109: 481-489.
Very cool! - I will have my high school AP Biology students read this.
ReplyDeleteI wonder if any studies have been done on whether crows acclimate to humans - maybe there is a sort of tipping point in the intelligence of urban species, before which their vigilance increases with intelligence because they fear humans, and after which their vigilance around humans declines because they are aware that no harm will come from humans or from other threats like open areas or leashed/fenced in dogs.
ReplyDeleteGreat analysis of the experiment!
ReplyDeleteIt might be interesting to see if crow vigilance prevents an entire group of crows from foraging, or if some individuals watch while others choose to eat. This sort of behavior would optimize the foraging capacity of the flock, rather than each individual. A small observational study could accomplish this goal.
ReplyDeleteI think your study is extremely interesting and I particularly appreciate your further delving into the issue of crows once your pilot showed you that crows ate your eggs instead of your original target species (raccoon). It would be interesting to see how differently (or similarly?) raccoon and crows react to human activity. Put together with Conard's blog, I think you raise some good questions about what the meaning of "natural habitat" is and whether urbanization is as strongly negative towards animals other than humans as we are intuitively trained to think.
ReplyDeleteI agree with Amrita and Toni here, that it would be interesting to see how crow response varies across campus, since some areas seem to provide the same environment (good tree coverage without any obstructions on the ground.) With no roads nearby but increased foot traffic by students going to and from the dorms, would crow vigilance increase? This is also an awesome example of how the unexpected can create a new, but still interesting, study.
ReplyDelete