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Cats, Lies, and the Smithsonian Catbird Study

April 10, 2011

Like many of us in this neighborhood, I like birds and animals, including cats. So I was dismayed to see the recent  SF Weekly with a huge cover picture of a demonic cat. It contained an article by the controversial journalist Matt Smith, using as fodder a press release from the Smithsonian about a study done in 2004.

That study implied that cats are the main cause of death of small birds. It’s become a talking point for those advocating stronger measures against feral and outdoor cats.  But did the study’s results justify it? No way.

It sounded pretty bad. According to the website of the Smithsonian, “The main determining factor was predation, which accounted for 79 percent of juvenile catbird deaths within the team’s three suburban study sites. Nearly half (47 percent) of the deaths were attributed to domestic cats…”  This makes it sound like cats killed a huge number, over a third of the birds. What huge number was it, actually?  Well, six.

The study followed 69 young catbirds (the name is a coincidence, it has nothing to do with cats really) by putting tiny radio transmitters on them. In the five months of the study (May-September 2004),  42 died from predators, glass-panes, or disease. Cats killed only 6 of them.

Here’s the real picture:

WHAT THE RESEARCHERS DID

Working in three Maryland suburbs filled with homes and gardens, the  researchers tagged a total of 69 baby catbirds with tiny radio-transmitters. They tracked the birds until the bird was found dead, or its signal disappeared, probably meaning it had moved on.

Of the 42 that died, the researchers figured 33 were killed by some kind predator. For 19 of them, the researchers thought they could make a good guess at what kind of predator got the bird. Besides the 6 actually killed by cats, they thought another 3 probably were because they were found headless. But it turns out that many hunting birds (owls, hawks, for instance) also bite off the head first, so assuming cats did it is a stretch.

Unfortunately, when the study was published, the abstract (the summary that’s the only thing most people read) said:

Why is this misleading?  Two reasons. First, it includes the headless birds that probably were killed by something else. Second, it compares the number with “known predation events” — which just means the number of cases where they guessed at who killed the catbird. What they should use is a comparison agains the whole bunch of tagged birds: 6 out of 69. That would be 8.7%.  The graphic below represents the real story.

 

And since it’s in the nature of predators to go for the weakest and slowest, it’s more than likely that if the cats didn’t get them, something else would have… an owl, a hawk, a rat, or disease.

What really hurts our birds is destroying their habitat. Everywhere in the city, bushes and thickets are being removed, even in the so-called “Natural Areas.”  Some ground-nesting birds like the quail are particularly affected. It’s easy to blame cats, especially feral cats. Though cats do hunt birds, they’re far more likely to get rodents. But small birds are prey to everything that hunts: hawks, owls, coyotes. They need places to hide and nest.

Not that I’m particularly advocating outdoor cats. Quite aside from the whole bird issue, they are themselves at risk from predators, whether with two legs or four legs or four wheels. As their reflexes slow with age, they face greater odds.

[A longer version of this article appeared on the Sutroforest.com site.]

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