Cats and birds

This blog post by Maximilian Auffhammer about how much people (de)value the view of a wind turbine from their house mentions another oft-heard critique of these big windmills, namely that they kill birds. Which they do: roughly 250,000 birds per year, according to an estimate from about ten years ago. More turbines today, so probably more dead birds. Sad! The same source puts the number of birds killed annually by cats in the United States at... hmm... 2.5 billion, or 10,000 times the number killed by windmills.

How is that possible... 2.5 billion?! Let's do a back-of-the-envelope calculation. There are an estimated 60 million pet cats, and perhaps 50 million feral cats in the United States. Many of the pet cats are outdoor cats, eagerly prowling the neighborhood picking off songbirds just for the fun of it. But let's stick to the feral cats. My lazy, inert indoor cats consume about 5 ounces of cat food per day each– pure meat and calories. A typical junco, the abundant, smallish sparrow that is the most common bird in my suburban neighborhood, weighs about 0.67 ounces, and some of that is feather and bone. But anyway, let's round up to a somewhat more substantial bird, 1 ounce of meat apiece. Assuming an active feral cat needs as much meat as my inert cats, that's 5 birds per day. For the U.S. feral cat population, that comes to 5 x 365 x 50 million = 91 billion songbirds per year. Of course, they will eat some mice and rats and garbage instead of birds, and some will be fed by well-intentioned human enemies of songbirds. But 2.5 billion seems utterly plausible. Out of a total bird population of... 10 billion? Busy, hungry, cruel, destructive kitties!