After dogs, cats are the world’s most beloved pets. But when they live outside human care, they can turn into one of the deadliest invasive predators on Earth.

Feral cats look identical to the cats that nap on couches and play with yarn. But their lives are very different. A feral cat is a domestic cat that has been abandoned—or was born to cats abandoned generations ago. Over time, these animals adapt to surviving in the wild. They avoid humans, form colonies, and become highly efficient hunters.

Globally, feral cats are linked to the extinction of 33 species and remain a leading threat to 8% of the world’s most endangered birds, mammals, and reptiles. They don’t just hunt for food—they often kill more than they eat. On islands, where native species evolved without mammalian predators, cats are especially devastating.

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Hawaii is one of the starkest examples. Cats arrived with European colonizers in the 1800s and quickly multiplied. Today, an estimated 2 million cats roam the islands. Native seabirds such as the Hawaiian Petrel and Newell’s Shearwater nest in burrows on the ground—making them easy prey. Researchers have documented cats dragging chicks from burrows, leaving carcasses uneaten, and moving on to the next victim. Populations of some seabirds have collapsed by over 90% in two decades.

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This has sparked a fierce debate. Conservationists argue that predator control—trapping and in some cases euthanizing feral cats—is essential to save endangered birds. Cat lovers, however, call such measures cruel and advocate for Trap-Neuter-Return (TNR) programs, where cats are sterilized and released back into the wild to gradually reduce populations. Critics of TNR say colonies persist and continue to harm wildlife. Both sides claim the ethical high ground: protecting vulnerable birds or protecting cats from mass killing.

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The dilemma is clear: Should feral cats be eradicated for the sake of biodiversity? Or should more humane methods prevail, even if they take longer? The answer depends on where you place the greater moral weight—on saving threatened species, or on sparing the cats.