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50. Cats on Birds
- University of Minnesota Press
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A N I M A L S A N D U S 195 50 cats on birds a more insidious side One might think that the authors who titled their scientific paper “Urban Bird Declines and the Fear of Cats,” published in Animal Conservation in late 2007, intended the title to be somewhat tongue-in-cheek. Indeed, the title does mislead in the sense that birds, to our knowledge, do not “fear” predators in the same way that we might feel fear if being stalked by a grizzly. But the authors were serious and bring up an important issue regarding the catbird debate that I think has been overlooked. First, let’s be clear about the natural history aspects of this issue. Yes, cats kill birds and rodents, and that is what they did before they were domesticated. So, it seems “natural” to a lot of people. However, be equally clear about this: our birds did not evolve in an ecosystem with a huge population of feline predators of that size and ability. Neither did they naturally encounter pesticides or pollution. So, cats are nothing short of “ecological pollution” when they are allowed to run outdoors and kill native wildlife. House cats kill a million birds a day on average in the United States. This number seems high until you consider that there are close to ninety million cats in the United States. Many of these are allowed to roam, and especially in the southern parts of the United States they can be feral for most of the year and do tremendous damage. There are even misguided programs that promote releasing spayed and neutered cats into nature. Work by Stanley Temple’s lab in Wisconsin estimated that at least seven million birds were killed by domestic cats each year in Wisconsin alone (and the upper-end estimate was over two hundred million). But there are naysayers. Some websites challenge the data from Temple’s lab and say that the estimates are too high (the fact that there are zero scientific data to the contrary is apparently irrele- t h e t h r e e - m i n u t e o u t d o o r s m a n 196 vant) and cats don’t do as much damage as commonly stated. But could there be more “damage” than direct killing of birds? Enter the “fear of cats” from the title of the paper. The three authors of the study focused on England, where the number of cats has increased from four million to eight million in the past thirty years. The authors noted that in some places in England the documented effects of cat depredation on birds are relatively low. So, that would reinforce the notion that cats don’t do as much damage as some people think. But there’s a twist or two. First, scientists have found that predators (e.g., cats) have an indirect, sublethal effect (fear) on prey species (e.g., native birds). In areas with high predator density, which one could argue is everywhere because cats add to the predator community, aspects like reproductive success declines and higher levels of stress lead to early death. For example, species that normally nest relatively near the ground may choose higher sites to avoid cats, and these higher nesting sites may be either less safe or less available. So, even if a cat does not directly kill a bird, it can have serious negative consequences on bird populations owing to lower numbers of young produced and higher death rates. That is pretty insidious, and considering that at least in England the ratio of cats to birds ranges from 35:1 to a low of 1.5:1, depending on the species of bird, birds have a lot of cats to be afraid of! Although the authors of the study have no direct information on the “fear” factor, they used models to predict its effect. They concluded that given current cat densities in England, one could see a 95 percent decline in bird populations owing simply to the fear factor. The results make sense. In some areas of England, cats kill relatively few birds, which leads some to believe the “cat menace” is overstated. But if you find an area with lots of cats, chances are you’ll find many times fewer birds than you’d find in areas without them. In such an area, if you go out and track cats, you’ll see...