E47: Okay, but how bad are cats for birds?

Scott Taylor: It's 1894, and there's this tiny windswept chunk of rock called Stephens Island sitting out in the strait between New Zealand's two main islands. A brand new lighthouse has just gone up there. The keepers move in with their families, and somebody brings a pet. As the story goes, her name was Tibbles. More on her in a second. Living in the rocks on that island was this small little flightless wren, palm-sized, olive brown, with a pale stripe over each eye. It didn't really fly, it just ran around at dusk, low and fast between the rocks, kind of like a mouse. New Zealand actually used to have a whole bunch of little wrens like this, but about 700 years ago, Polynesian voyagers showed up with a stowaway, the Pacific rat. These birds had never had to deal with a mammalian predator, so it was pretty catastrophic. One by one, they disappeared from the mainland, except this one, which hung out on this one little island with no rats. Back to Tibbles. She was a cat, and as the story goes, she was pregnant. One of the assistant lighthouse keepers, a guy named David Lyall, was really into birds. A cat starts bringing him these little brown wrens. He sends a few off, and suddenly, some of the biggest names in ornithology are racing to describe this weird new species. It ends up named after him, Lyall's wren. Meanwhile, there are kittens, and pretty soon there are feral cats all over the island. Over the next year, dead wrens continue to appear, and before you know it, they're all dead, gone. You might have heard that Tibbles the cat wiped them out all by herself. The real story is a little messier. It was a whole population of feral cats over a few years. But either way, Lyall's wren was gone in a very short amount of time, almost as soon as anyone knew it existed. Okay, that's an extreme case. Tiny island, bird with zero defenses. But cats aren't just an island problem. I'd hazard a guess that a lot of you have heard the phrase, cats kill billions of birds. Which brings us to the topic of today's episode: Okay, but how bad are cats for birds? To help us unpack that, I'm joined by Dr. Pete Marra. Pete runs the Earth Commons, Georgetown University's Environment and Sustainability Institute, and he literally wrote the book on this. It's called Cat Wars. After the break, Pete walks us through where that billions number actually comes from. We get into why a well-fed house cat that's just being a cat still hunts, and whether that little bell on the collar does anything at all. And we talk about why this is one of the most heated fights in bird conservation. Stay tuned.

Scott Taylor: Welcome back to the podcast, everyone. I'm really excited to have Pete with us today to talk about cats and their impacts on birds. Thanks for joining us, Pete.

Pete Marra: My pleasure. Thanks for having me.

Scott Taylor: Yeah, I know this is a topic you've thought a lot about, and I would hazard a guess that a lot of our audience has heard the phrase cats kill billions of birds. And I'd love if we could start with just where that number came from and how much of that is like pet cats versus strays and ferals.

Pete Marra: So that number comes from an analysis that we did. 15 years ago, almost now. Time flies. Yeah. It was really a postdoc, Scott Loss, who I had hired, brought on board to try to estimate the overall mortality from multiple threats, direct mortality from anthropogenic threats. So it wasn't just cats at all. It was windows, infrastructure, buildings.

Scott Taylor: Mm-hmm. I remember when it came out.

Pete Marra: Cell phone towers, automobiles, electrocution from wires, and cats. And we were also hoping to do pesticides. So Scott came on board this is when I was at the Smithsonian Migratory Bird Center and began this incredible literature search to try to understand you know what are what's available what data are available in the literature for each of these various threats and we wanted to ultimately try to compare them like if you were able to come up with the best estimate possible with uncertainty this was the piece that we added because most of the other estimates out there didn't have uncertainty associated with them so they were just like okay cats kill about a billion million birds period we didn't know that was plus or minus two million or what the deal was.

Scott Taylor: That seems important if you could have some uncertainty.

Pete Marra: Really important. And the cat one was the first one we did. And when I saw some of the first estimates from that systematic review process and the quantification and uncertainty, it was like... whoa, this is pretty significant. Let's check the numbers. Let's really think about this carefully. Put a lot of time and effort into it. And he did separate the mortality due to owned cats that are let outside versus unowned cats that might be at a colony. Those are sort of the two different pieces to the puzzle that we tried to separate.

Scott Taylor: How different were those? Like how different was it between owned cats and these kind of more feral colony dwelling cats? Was it quite a big difference?

Pete Marra: Yeah, it was almost, I seem to recall, like twice as high for unowned cats. But it was still quite significant for owned cats. You know, owned cats fed, obviously, and still let outside. And there'd been a lot of papers on this already that cats kill not just because they're hungry, but they also kill because it's an innate problem. Tendency that they have a behavior that they chase a feather if it's dragged in front of them they chase a little ball that's tossed across the room and they do the same thing because they're curious and they want to explore and they want to satisfy these behaviors that they have to satisfy.

Scott Taylor: It's not that surprising. I guess like the number itself is surprising that it's so large, but when you think about how many people have cats that they let outside, and then also, you know, I grew up in rural Ontario where having outdoor cats, like, you know, a colony of cats living in a barn was just what people had to control rodents, you know, on a farmyard setting or something like that. And so cats eat birds, but a lot of other things eat birds, right? So birds have other predators, hawks eat birds. And so... What makes a house cat a different kind of predator than most of the things birds, at least in North America, evolved beside? Because I think one thing people will say is like, well, cats are just part of the environment. But how is that wrong for North America and for places like New Zealand and elsewhere?

Pete Marra: Yeah, so it's a great question. And cats are being just cats. That's exactly what the problem is. That's exactly right. And they are 100% carnivorous. They are predators. They are doing what they should be doing in some ways. In other ways, they are at densities that are completely unnatural to what the local ecosystem can support from a large predator, a mesopredator, a mid-level predator. They are a domesticated species like a horse, like a donkey, like a bunny, a non-native bunny. They are not native to any ecosystem. So when they're put into an ecosystem that's not used to a species like that, or at densities that are extraordinarily high, the local ecosystem usually suffers negative impacts. And we've seen this over and over again, all over the earth. So that's the big difference. So yeah, they're doing what they should be doing, but you could argue that with anything that humans do when they destroy the environment. And that's not an excuse.

Scott Taylor: Right. Like outdoor cats also kill small lizards and snakes and small mammals and all sorts of things. So they are not just having impacts on birds and pretty catastrophic overall.

Pete Marra: Yeah, in fact, the impact on mammals is three or four times higher than it is on birds. That's really clear. Yeah, small mammals. And it's hard to quantify the insect impacts, but we know that they're taking dragonflies and butterflies and all sorts of things. Yeah.

Scott Taylor: The insect impacts. I never even thought of that, but I have seen just this summer, I saw a cat hunting dragonflies and it didn't even register how that could have an impact, you know, and it certainly could. I So, I mean, there's this direct mortality that you mentioned. So cats are killing a... So is it in the billions? I don't remember the exact number.

Pete Marra: It's absolutely in the billions, and it's more than any other source of anthropogenic mortality. And when you look at other estimates like in Canada, it's also, it's not in the billions, but there are many fewer cats in Canada, but it's still the highest source of direct mortality. And this is true in China now. We helped with an estimate in China, same sort of thing. So relative to other sources of direct mortality, cats are still the highest mortality source.

Scott Taylor: Yeah, that's really important to think about. Like they're the highest source of direct mortality when a lot of people are investing in, you know, putting things on windows and turning lights off at night. And those all have impacts and they're good things to do. But then people often seem less prepared to kind of... deal with the cat issue, That's a hard number even to wrap your brain around, like how we still have any birds.

Pete Marra: And the thing to remember, you know, one of the controversial thing that always comes up is, you know, how is that possible when we think we have 10 billion birds in the, you know, in North America, in the U.S.? The thing that people have to remember is that number 10 billion is also a rough estimate without uncertainty. But it's a measure that's taken just pre-breeding, just before animals start to breed. So like in May is a single static number. And what happens then is that birds breed and there's a contribution of multiple... nests of young to the population so the population of animals changes throughout the year as animals produce as they die numbers change and so this is the complexity that a lot of the cat people don't understand because they're not biologists that the number of that 10 billion could triple quadruple as animals are recruited in the population towards the end of summer And then some die. But that mortality estimate comes from throughout the year. We're using estimates of mortality throughout the year to figure out how many birds are killed by cats throughout the year. That's the estimate. But it's not based on the static number. It's based upon the best estimates we have for where studies have gone out in estimated mortality.

Scott Taylor: Yeah, so the lower range is 1.3 billion. The upper range is 4 billion a year, which is just incredible to think about. And that's direct mortality, but cats have other impacts on birds. So yes, they directly kill them and either eat them or not. Like you said, they'll kill just because they like to chase things that are moving. What other kind of indirect impacts do cats have on birds?

Pete Marra: Fear and so there was a wonderful study where someone took a cat stuffed cat and put it near nests and looked how it changed the behavior of the adults at the nest and it really changes the behavior birds are you know aware of a predator near the nest and that changes how often they come in to feed the nestlings how often they incubate and so there have been several studies that have shown that there's a really important consequence of just being present outdoors near a nest. So that's one really clear way that they indirectly influence birds in particular.

Scott Taylor: Yeah. And so some of the work, people often ask, how do you know? But there are studies that track specifically what individual cats are doing. And you've done some work on gray catbirds in DC neighborhoods to find out what kills them. And so what came out of that research?

Pete Marra: Yeah, this was a study with a master's student, Anne Balogh, who was working on our project Neighborhood Nest Watch. When I first moved to DC in '99, and it was clear that cats were a part of these ecosystems in urban and suburban settings. They were probably a big source of mortality. And so Anne began to study reproductive success in gray catbirds, a really common species here in DC, and started looking at reproductive success and realized that in areas where there are really high densities of cats, in the town I live in actually, Takoma Park, reproductive success was close to zero from these catbirds because cats were preying on the fledglings and the nestlings in the nests. In areas where there were almost no cats, there was really high reproductive success. So this was one of the first papers that we published on cats and it really got me thinking about the cat issue in these situations a lot more than I had before But that cat paper was a really important start in trying to understand this really important factor that drives success of birds in these systems.

Scott Taylor: Yeah, 100% mortality, up to 100% mortality of fledglings and even nestlings is, yeah, that's a huge impact on local populations. Obviously, you can have, then you're only going to have catbirds as long as the adults can live, which if they can't reproduce themselves, there's not going to be catbirds anymore. What sorts of conversations have you had with cat owners and in the context of your work and your findings and what are the typical responses?

Pete Marra: Yeah, you know, it's changed over time because I can almost have better conversations with my neighbors to try to talk to them about, you know, what we know, what we don't know. This is not about hating your cat. You know, cats are incredible. Animals they're really fascinating to watch you know as a behaviorist I really love watching cats I mean it's like I get it I really do get it.

Scott Taylor: Yeah, they are fascinating little creatures. Mm.

Pete Marra: They're fascinating. But what I try to tell people now is any little thing you can do can probably help. Like don't let your cat out at night. It's not good for your cat. Most of these cats live half the life that they would indoors only. And then I usually bring up the disease issues that they potentially carry. But then I just say keeping your cat at night is not only safer for your cat, it's safer for everything else in dusk and dawn when birds are really active and they're particularly vulnerable. And then, you know, make this your last cat that you have outdoors. You know, these conservation issues, I seem to feel like they're on these 20-year time frames. The cat issue is not one of them. But I still feel like, you know, if people think of this as their last cat outdoors, they start to be a little bit more open to change. And I can feed them data in ways or information in ways that's non-threatening. I think at the end of the day, we're more likely to change minds and change behaviors than we would be if we turned it into an aggressive shouting match. And so that's why I try to talk to people rather than argue with them about these issues. It's just not worth it. Part of the issue is that some cats are just really hard to keep indoors because they need stimulation. I mean, any animal needs stimulation. If you're an animal owner, a pet owner, it's your responsibility to create an atmosphere for that animal, whether it means lots of walks. Some breeds of dogs, it means you've got to run them. Or if it's a cat, you've got to stimulate it with that feather toy or whatever it happens to be if it needs that. That is called creating an enrichment environment for the animal. Zoos do this all the time for their animals. They have to create ways to keep that animal's mind moving. And if you're a cat owner, ideally you would create these various trees, cat trees or whatever, to keep it indoors and keep it stimulated. Just opening the door and letting the cat go out is not the answer to that.

Scott Taylor: Yeah. Yeah. I think about that in the context of like getting a working dog breed. I really am. I've always thought dog breeds are very interesting because artificial selection is insane and leads to all these characteristic behaviors. But yeah, you're not going to get a border collie and keep your border collie in your house forever. Like you're going to take it for walks. You're going to give it jobs. Maybe you're going to take it on like a... little herding exercise it's so interesting. Again, people just think about cats differently. Like, oh, I just have a cat and I keep it in my house and let it outside for enrichment versus like doing your job as a pet owner is quite interesting to think about. When you have these conversations with people, at least I've had these conversations with people. They often say, well, my cat doesn't do that, which we know from lots of data is not true. Even overweight cats or cats that humans might not perceive as being very... athletic can still kill birds and mammals and other things. So I'm curious about what actually reduces the number of birds cats kill because there's lots of things people try. Some people put bells on their cats, but I'd like to hear if that actually works or not.

Pete Marra: Bells do not work. Turns out birds don't know what a bell is.

Scott Taylor: Oh, shocking. I've always thought, like, why would a bird hear a bell and think, like, danger the way that... Okay. Yeah.

Pete Marra: Right, exactly. It's just not. Oh, it's dinner time. No, it doesn't work that way. Yeah, no, birds don't really recognize the bell. So the bell is not the solution at all. There are other things like there's a bib that you can put on cats to sort of like stop its paws from moving forward. Or there's a collar that people sometimes also use on cats. And to me, I never advocate for those things just because it's not only about what the cats are doing to other animals. Turns out that bib and that collar, does not stop a tire from running over the cat's head.

Scott Taylor: There you go.

Pete Marra: The solution is a leash. And I feel like there are many more people walking their cats on a leash. You're not going to go out for a jog with your cat. I get that. But you could go out and bring your dog for the run. But bringing your cat out for a walk with a leash is a real thing. People do it all the time. There's also catios and so you can if you have a screened in porch or people are actually building things called catios to get your cat to get the stimulation it needs just by watching it doesn't have to kill things out there it can just watch those things out there so there are multiple ways to do this without just allowing your animal to be animal to be your pet to be free range and pets just do not should not be in the same sentence when our book came out, Cat Wars, I was at a restaurant and it was like the day after. And for some reason, the waitress and I got into this conversation about cats. I guess it was on my mind. And she said, yeah. And she said, I only walk my cat by a leash. And I said, you're kidding me, because I hadn't really encountered this too often. And she said, yeah, my first cat got hit by a car and my next cat was killed by a coyote. And she said, I'm not going to let that happen again. And so it was such a sad, but such an awesome thing to hear that people's minds can change, but sometimes it just takes direct experience and direct exposure to what this means to lose an animal you love and to understand that. So, yeah.

Scott Taylor: Yeah, that's a really good point. Sometimes it does take those really hard experiences to make decisions and change the way you behave. We haven't talked about trap, neuter, return work either, but what are your thoughts on that? So a lot of people say, you know, in managing these cat colonies, if they can't reproduce anymore, that's sort of a solution that eventually they'll just, there won't be any anymore. But what are your thoughts on trap, neuter, return work with respect to cats?

Pete Marra: Yeah, it's a failed policy. It doesn't work. There's no data out there from an open colony perspective that suggests that it does work. So there are one or two papers out there with closed colonies. That means there's no emigration or immigration into the colony allowed. So yeah, over time, those animals will die. That happens. The other problem with these, in most cases, is you've got people, just amateurs, that care about cats that are going out trying to trap them and neuter them. And they are never successful in trying to capture all the cats. You know, and I know, because we work on animals in the wild, it's very hard to catch 100% of the population. There's always a certain percentage of that population that has outsmarted us, even though we don't ever want to get outsmarted by our redstarts and chickadees. We always are.

Scott Taylor: I've tried to capture cats. Yeah.

Pete Marra: And the same is true for these folks. You need to catch at least 75% to 90% of the population of adults to ensure there isn't enough reproduction to maintain the population size. And if you don't get to that level, there's ultimately going to be more recruitment than mortality is offsetting. And so in these colonies that are maintained, they just don't ever catch all those cats. And so that's always going to be one of the problems. And these colonies end up persisting. Those cats end up continuing to kill other species and disperse. And there's introductions. And there, it's unbelievable how many TNR colonies are in cities like Baltimore and some of these other cities out there or just in the mid-Atlantic area. We're not going to get rid of them overnight. We're just not. And so I feel like I want to take a practical approach to how we do this. And so I have a zero tolerance for TNR colonies in natural areas, in any natural area. We might have to leave them in some of these other areas until we get rid of them from these natural in the other cases if we're going to leave cats in these urban areas we need to make sure we're getting our neutering up to a certain level we need to make sure those cats are protected and safe we need to make sure they're vaccinated that's the other problem with you know in many of these cases These cats are caught, they're vaccinated once, and it's hard to capture them again to update their vaccines, which, you know, if you have a pet, you've got to update the vaccinations in these situations. Cats are now, speaking of disease, cats are now the number one domesticated species to transmit rabies to humans. Used to be dogs, but now it's cats.

Scott Taylor: Really? Yeah. And so this is a human-created problem. I mean, we haven't talked about the fact that places like New Zealand really have adopted zero-invasive policy. And cats are an invasive species in New Zealand, So New Zealand is just trapping and killing all of their invasive species.

Pete Marra: Is it okay to kill one animal to save another? It's an ethical question. It's theoretically fascinating, but it's one we grapple with all the time as conservation biologists. It's where we're at. It gets back to this question of, since we've manipulated environments and moved things around the world, To what degree do we need to manage and maintain? And what do we maintain to? What is the state we're looking to sort of get to?

Scott Taylor: Yeah. That baseline question is a challenging one. Yeah. Yeah.

Pete Marra: Yeah. And so I absolutely applaud New Zealand for what they're doing. It's a very hard decision that you have to make. Otherwise, you're going to lose these extremely fragile species that, you know, if they look back in their history, they've lost many species already due to cats. Stephens Island wren is one of the classic examples of a species that was lost to extinction. And there's probably many more that we don't even know existed. Because we haven't quantified them. Australia is doing the same thing. Australia has created this cat cull. Because they have many species of not just birds, but lizards that they're trying to protect that are on their last legs. And cats are the primary threat to those species. The cat people will argue that, well, cats aren't having population level impacts. And of course they are. Cats have caused the extinction of 63 species around the planet. 40 of those are birds. 20 of those are mammals. Three of those are reptiles. That is a population impact to zero. So, you know, evolution, thousands of years of evolution created those species. And then a cat is introduced and those species can be gone in 20 years. So, yeah, we've got to make those tough decisions. New Zealand and Australia are doing exactly what we need them to do. If anything, they need to do a little more aggressively. It's just a really challenging problem.

Scott Taylor: Yeah, it's a challenging problem. And to see it on the ground is also just like wrapping your brain around like, you know, to this finding all the predators and removing all the invasive predators. That's a hard, very hard thing to do. But when you're in New Zealand and you're in a place that's predator-free now, it is completely different. It's full of bird life and... it was pretty incredible to see the difference between these predator-free areas and areas where they haven't yet fully eradicated cats or weasels or possums.

Pete Marra: You know, Hawaii is doing the same thing because cats are such a problem with birds in Hawaii. And so with nesting seabirds, they've actually fenced off several areas that cats were invading and killing, you know, endangered seabirds, nestlings. And so they've done the fencing in Hawaii too. This is where we're at. It's a battleground. It's just insane.

Scott Taylor: Yeah, I take students to the Galápagos, and it's so depressing to be walking along the edge of a crater that you know within the crater, Galápagos petrels nest, and you just see cat everywhere because the feral populations of cats are so high, and they're preying on these birds. And we saw several these birds only come to land once a year, if that, to breed. And then these introduced cats are killing them. I know on Floreana, they're doing a predator removal because they're trying to reintroduce the Floreana mockingbird. And they certainly don't want to reintroduce the mockingbird until there are no cats left since there's just a handful of those critically endangered birds left.

Pete Marra: Yeah. And it's amazing. This story has been played out multiple times on islands where when you remove the invasive species, whether it's cats or goats or sheep, the birds come back. The numbers rebound really rapidly. So it's a really important management tool. And there's some groups out there like Island Conservation, which is just doing remarkable work in this area.

Scott Taylor: Mm-hmm. What makes you optimistic that people's behavior is going to change? I'm sure, as you said, you've had difficult conversations with people. You've received death threats for this work. It's just like... I don't know. I'm sorry that you've had to go through that because it doesn't make any sense. This is a conversation we should be able to have. What makes you optimistic about the future of this problem and this future of this challenge for birds?

Pete Marra: Yeah, I'm hopeful is what I'll say. I'm definitely hopeful that people are going to get it. And despite the death threats, it's been a welcoming way to sort of expand the conversation about nature. It's realized I've you know I've never I've been a big advocate for science and I really feel strongly that we need to be defenders of science and how much science we do and I've always found it problematic a lot of conservation biologists will say or policy people we don't need any more science so I push back on that all the time except on the cat issue On the cat issue, I don't think we need a lot more data or research on what cats are doing. What we need to do studies on is how and why people will not accept the data and not accept the science. So I feel like we need to study the people on this issue, the ones that are sending out the death threats, the ones that are... You know, there's such strong cat advocates that they forget about natural ecosystems and what really, how complex these ecosystems are. I'm more hopeful that once we move into that direction and I feel like people are starting to realize that conservation needs to include really thinking carefully about the human psyche and how to change behavior. And I'm seeing it. I totally am. When I first started getting into this, I never saw people walking cats. I'm seeing people walking cats. I never saw catios. I'm seeing catios. And so I do feel like it is changing. I'm seeing some towns that are saying no to TNR, trap-neuter-return. I'm seeing some international pushback on this. I just feel like it's I choose to be positive and choose to be optimistic that this is going to change over time.

Scott Taylor: Yeah. And it is funny. Even people who love birds still have outdoor cats. It doesn't, does not compute.

Pete Marra: I think it's a sense of freedom that they sort of live vicariously through their cat.

Scott Taylor: The cat's on its little wander and somehow they get something out of that. Man. Yeah. Oh, man. Yeah.

Pete Marra: Their mini Serengeti in their backyard. The number one thing that animal rehab people deal with are cat injuries caused by cats. And there's some incredible documentation at all these rehabilitation centers on the mortality and the impacts of cats. And even if a cat just scratches a bird or bites a bird and the bird still seems alive, that bird will likely die because of infections.

Scott Taylor: We've reached the part of the show we call That's B.S., or That's Bird Stuff, where we give our guests an opportunity to debunk a myth that ruffles their feathers. So Pete, what do you want to call BS on?

Pete Marra: Cats are not native to any ecosystem anywhere in the world, domesticated cats. They're a separate species, but they don't occur in any system as a native species. And this is something that people really have a hard time with. It's like, well, where'd they come from?

Scott Taylor: Yeah. Bobcats are native to North America, but they're very different than a domestic house cat. Yeah.

Pete Marra: That's right. Much lower densities. It's like saying, okay, well, where does a basset hound, where can I go catch a wild basset hound? Off some small island in Scotland? Basset hounds don't exist in any ecosystem anywhere either.

Scott Taylor: Can you imagine? Yeah, exactly. Well, I really appreciate all your insights on cats and I appreciate you taking the time to chat with us on the podcast today. I'm also hopeful and I guess we'll see where things go, but thanks again for taking the time.

Pete Marra: It's a lot of fun, really. Thanks for having me.

Scott Taylor: Of course.

Scott Taylor: Birds are dinosaurs, and around here we like our snacks, so we end each episode with a dinosaur nugget. Today's nugget is, cats are the biggest human-caused source of direct bird deaths in the United States, somewhere between 1.3 and 4 billion birds a year, plus billions of small mammals. And domestic cats aren't native anywhere on Earth. Pet or feral, a cat outside is hunting, and that bell on the collar doesn't help. Even just hanging around a nest, cats can change how often parents feed their chicks. Trap, neuter, return colonies don't make the problem go away. But there are real fixes. Keep your cat inside, especially at dawn and dusk. Try a leash or a catio. And maybe make this your last outdoor cat. On islands where cats get removed, the birds come back. That's a wrap on this week's episode. Okay, but how bad are cats for birds? If you liked this episode, let the cat out of the bag. Rate, follow, and share with a friend. We'll catch you next time. Byeee.

This transcript was generated using AI-assisted transcription and may contain errors or omissions. Please refer to the audio or video episode for the most accurate representation.