Millions of birds are being killed by outdoor cats

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I’m Nicole Mortillaro, and I am a proud "crazy cat lady." I have three of the little furry psychos and I love them to death. But I do not let them outside, because I know they can impact the environment — but also because I want to keep them safe. So what exactly is the impact of letting them outside unsupervised? And why do people do it?

This week:

Cats are incredible hunters. They are ambush predators with incredibly flexible spines, sharp claws and insanely sensitive hearing, as well as night vision. They are stealthy and agile. They were made to hunt.

Which is why letting them outside to roam freely poses a danger to the local environment. 

I used to live on a street where an outdoor cat was a prolific hunter. Numerous times I saw her trotting away with a bird in her mouth. It was mostly house sparrows from a nearby bush they frequented. I started calling it the "murder bush." 

So I wondered: How many birds do cats kill each year?

First of all, it’s hard to get solid numbers on cats alone, but it’s estimated there are roughly nine million cats in Canada, making them the most popular pet in the country by some accounts. 

It’s also difficult to get a good figure of how many birds they kill a year. Some estimate that upwards of 100 million birds in Canada are killed each year by cats. An estimate conducted in 2013 brought that to between 100 million and 350 million.

But a recent study out of the University of Guelph did a new estimate with more recent data than the 2013 one, and it put the number of birds killed by outdoor cats between 19 and 197 million. 

Yes, that’s a big margin, but co-author Ryan Norris said that’s because each factor taken into account has its own margin of error, which is just exacerbated as you put them together.

"Coming up with an estimate like that is … let's say a 20-, 30-step process," he said. "There's all a bunch of numbers that go into it."

But no matter the figure, that’s a lot of birds. And that’s just in Canada.

Norris noted the type of birds killed varies depending on location. In the city, it’s more likely that the cats are catching native species rather than migrating birds, which Norris said is almost preferable.

"Cats are keying in on things like breeding birds," he said. "And, you know, nests are easy to find for cats."

In places outside of cities, the impact on wildlife can be greater, with cats also hunting smaller mammals, he said. These cats could even be going after endangered or threatened species.

He said the people he’s talked to in his various studies who let their cats outdoors have good intentions about the environment, but they don’t realize the impact their cat has on it.

The impact on unsupervised cats can be great. Feral cats are even believed to be behind the extinction of animals on several islands. 

One study estimated that each year, cats kill 1.3 billion to four billion birds and 6.3 billion to 22.3 billion mammals.

A 2017 study by Humane Canada, the national federation of Society for the Prevention of Cruelty to Animals and humane societies, found that 28 per cent of households with cats allow their feline friends to spend at least some time unsupervised outdoors.  

But why? Lee Niel, a behavioural biologist at the University of Guelph, is interested in answering that question.

"I think that there are the two sides. I think there are people who feel really strongly that cats need to go outdoors in order to have their needs met. And I think that there are also people who are less concerned about cat welfare," Niel said. "I wouldn't say that it's just one size fits all in terms of the reasoning."

For all the concern for the welfare of birds, there are also numerous dangers for cats outdoors. But in Niel’s research, that doesn’t seem to be a consideration for some owners. 

"When we sort of ask questions, you know, are you concerned about car accidents? Are you concerned about parasites, disease, these types of things? They were less concerned about the risks, and they felt more strongly about the benefits," she said. "So they felt that outdoor access was really necessary for their cat to have good welfare."

Niel also noted there’s a perception that cats are more likely to have behavioural issues if they’re kept indoors, though she noted there’s no clear evidence of that.

There are some ways you can still allow your cat to experience the outdoors or to use their hunting instinct.

One is a "catio," an enclosure for cats that allows them to enjoy the outdoors without posing a threat to wildlife. The downside is that these can be expensive.

Another solution is to "catify" your home. Cats like to jump and sit on high perches, which can keep them a little more active.

As well, cat owners should provide their furry friends with toys and engage with them in play, using things like cat wand toys to allow them to hunt. 

The solution to ending cat predation of birds and other wildlife is simple, Norris said.

"We know what to do with cats: keep your cat indoors. But it's getting people to do it," he said. "So how do you get people to do it? I don't know. It's a difficult one."

— Nicole Mortillaro

Check out our podcast and radio show.  In our newest episode: Seafarers of old saw kelp as a danger, marking it on maps to avoid it. Today, those maps are helping to restore kelp as a friend of the climate. Kelp sucks in carbon, and the Tseshat First Nation is working hard to bring it back around the Broken Group Islands in B.C.

In the previous newsletter, Inayat Singh wrote about a new carbon capability score — a more comprehensive way to score people's individual climate actions that goes beyond a personal carbon footprint, which has been criticized as a way of diverting responsibility for climate change away from corporations and governments. 

Yet readers of this newsletter still want to reflect on personal contributions to the solution. Douglas Holland said, "Granted, individuals do not have direct control over the actions or policies of corporations or governments, but we can exercise control over our personal choices, however constrained they may be by personal resources." 

That includes everything from food choices to recycling to transportation options and supporting certain government and corporate policies. 

"Measurement is less important than action," Holland wrote.

William Heath asked for a link to the climate capability calculator. A public-facing tool to calculate the score is not available yet, but this is something the researchers want to create in the future. The score is calculated through a wide-ranging survey that takes about 20 minutes to complete — and sometimes needs to be fine-tuned for its audience, such as people who live in areas with public transport, for instance, or live in a country where they vote for and can influence their leaders.

Write us at whatonearth@cbc.ca (and send photos there too!)

The U.S. National Oceanic and Atmospheric Administration (NOAA) is forecasting the re-emergence of an El Niño, a warming in the central equatorial Pacific Ocean, perhaps in June. This has repercussions around the world, typically raising global temperatures three months following its peak, which is forecast for November. 

Zeke Hausfather from Berkeley Earth, a non-profit climate analysis organization, looked at 11 separate forecast models, most of which suggest that this El Niño could be a particularly strong one, or even a “super” El Niño. The last strong one we had was in 2023-2024, which far surpassed the average, as seen above. And it was no accident that 2024 ended up being the warmest year on record, following 2023. That’s because as we continue to use fossil fuels, which pump greenhouse gases into the atmosphere, our global temperatures continue to rise, and an El Niño gives that an extra boost. That also means more severe weather around the world. The last super El Niño was in 2015-2016.

Hausfather did note that these are early projections, and that they could change. So we’ll definitely be keeping an eye on this in the coming months to see if this one nears or surpasses 2023-2024.

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