There’s been a huge amount of publicity recently about a 2004 study, publicized by the highly-respected Smithsonian. It implies that cats are the main cause of death of songbirds, and has become a talking point for those advocating stronger measures against feral and outdoor cats, even to killing cats. The SF Weekly published an issue with a demonic cat on the cover; even the normally thoughtful New York Times took the press-release at face value. Views From the Thicket, a great blog about Golden Gate Park, repeated the SF Weekly’s story.
But did the study’s results actually merit that kind of spin? We think not.
First, a disclaimer. We’re not advocating here for any particular feline policy. But we do respect all animals and birds, and even more, we favor a thoughtful approach to data. So we went back to the actual study. (You can read it here as a PDF.)
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 then tracked the birds alternate days until they found a dead bird, the detached transmitter, or lost the signal (when the birds presumably left the area i.e moved more than 5 km from the study sites).
At the end of the study, which was conducted between May and September 2004, they found that 42 of the birds died from various causes: predation, disease, glass panes, etc. Of that 42, they figured 33 were killed by some kind of predator. (They found the predator, or the bird’s remains with the transmitter attached, or they found the transmitter underground). In 19 of the cases, the researcher felt they could make a good guess at what kind of predator it was. Only 6 were killed by cats.
Another 3 were presumed to be killed by cats because of the state of the catbird corpse (missing a head). They thought this was a sure indicator of cat predation, but that’s been disputed. [ETA: A little websearching reveals birds of prey - hawks, owls, falcons, kestrels - seem to bite off the heads of their catch first. (There are slightly gruesome photographs.) If they are disturbed by people or competing birds, this could account for headless dead birds.]
We’ve shown this breakdown in the graphic above, where each circle’s area is proportionate to the number of birds it represents.
Unfortunately, the abstract of the study spun the same data differently:
Is this true? Yes… but does it suggest a misleading conclusion? Also yes. It seems to suggest that around 80% of deaths were caused by predators, and nearly half of those by cats. That is not true. They had only 19 ”known” predation events – meaning the ones where the researchers had a guess at Who Dunnit — compared to a total of 33 killed birds. [ETA2: And 42 dead birds. Taking the guess-work out, here's the story in graphic form.]
WHAT IF THERE WERE NO CATS?
So let’s do a thought experiment: If Maryland had no cats, how many extra catbirds would have survived?
If we looked only at the other 60 catbirds, (the ones that weren’t actually or putatively killed by cats) we find 33 died from other causes. They had a less than 50% chance of survival. So even assuming cats killed all the birds the scientists thought they had, only about 4 more birds would have survived.
If the birds killed by cats were in some way more vulnerable than the average — if they were louder, or weaker, or less wary, or already ailing — then they’d probably have been picked off by one of the many other predators. After all, that’s what predators do: They go after the weakest ones of a flock or herd. In that case, having zero cats in Maryland might have made no difference at all to the birds’ survival. Something else would have got them anyway.
So the effect of the cats seems to have been somewhere between 0 and 4 birds. Hardly the stuff of headlines. [ETA3: In any case, with such small sample sizes, statistical analysis is meaningless.]
HABITAT DESTRUCTION IS WORSE
What’s probably much worse for bird populations is habitat reduction. Responsible home-owners and dedicated park managers prune or remove trees, eliminate unsightly brush, trim bushes and plants, use pesticides to eliminate weeds and insects. All of those things destroys part of a bird’s world, maybe all of its world: cover; seeds; insects.
Here in San Francisco, and definitely in Mount Sutro Forest, habitat destruction is an issue. Thickets are being destroyed, because they’re “non-native” and “invasive” (the pictures here show an example the same place before and after this process).
Never mind that they’re the places where birds and animals can live and hide.