Discussion

Estimated Number of Birds Killed by House Cats (Felis catus) in Canada. 2013. Blancher, P.

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Estimating cat Numbers
Andrew Rowan on Oct 01, 2013 18:34:20

There have been a number of attempts to estimate bird mortality due to domestic cats over the past decade.  A feature of these estimates has been a steady increase in the estimated number of birds killed by domestic cats.  The most recent estimate (Loss et al, 2013) puts the toll of domestic cats at somewhere between 1.4 and 3.7 billion birds killed a year in the USA.  These numbers are based, in the first instance, developing a reasonably accurate estimate of the outdoor domestic cat population.   Unfortunately, there are no such data available.   Probably the most reliable estimate of the "owned" cat population are the surveys conducted by the American veterinary medical Association every five years (the most recent being in 2011).   A graph of the total owned cat population using these data is reproduced below. 

 

As can be seen, the rate of cat ownership per household is not increasing but the cat population is increasing because the total number of US households is increasing.  It is also evident that the point for the total number of owned cats in 2006 is higher than the trendline would predict and the point in 2011 is lower than the predicted number.  It is also relevant to note that the method used in the AVMA survey (use of Household Panels) produces estimates of owned dogs and cats that are higher by a factor of about 20% than surveys carried out by Random Digit Dial methods (Patronek & Rowan, Anthrozoos 8:199-205, 1995).  To date, nobody has yet indicated which number is more accurate. To further confound the situation, the AVMA survey data allows one to calculate cat ownership rates for each state and the number of cats per household varies by a factor of 2 (excluding DC and Hawaii which have very low cat ownership rates).

One then has to address the question of how many cats are outside, how long they spend outside and how many unowned stray and feral cats there might be.  It is evident from a variety of data sources that owned cats in the USA are not outside very much.  Surveys indicate that only about 40% of the owned cats spend time outside and a majority spend eight hours or less a day outdoors.  There is a strong upward trend in the keeping of owned cat indoors (mostly because owners perceive that the indoor environment is a safer place for their cat). We do not have similar data for Canada but the Canadian animal protection movement has been spreading the message that cats should be kept indoors.  

Finally, what can one say about feral/stray cats?  Unfortunately, one can say very little.   A study in Florida estimated that the stray/feral cat population was about 80% the size of the owned population.  However, a recent survey by one of my colleagues in Maine indicates that the outdoor stray/feral population may be less than 10% of the owned population (Maine has among the highest rates of cat ownership in the US).   Using average cat density numbers for different land use regions, one can estimate the stray/feral cat population in the USA as being around 35 million but this is purely a starting point and not a reliable estimate.   It does seem apparent that cold weather states (and states with significant coyote populations) are going to have lower numbers of stray and feral cats.  Therefore, it would seem reasonable to assume that Canada would have lower numbers of stray/feral cats than the USA (proportionately).  

Finally, the hunting behavior of cats is very asymmetric.  It appears that only a minority of owned cats engage in significant hunting activity and there also appears to be a tendency for cats to choose mostly ground prey.   

Andrew N Rowan, PhD

CEO

Humane Society International

arowan@humanesociety.org

Avian Conservation and Ecology ISSN: 1712-6568