The first sign of something amiss came on the night of November 18, 2018, when residents heard a commotion among the fairy terns. A white cat was chased out of the sanctuary. In the days and weeks after, Greenwell started finding dead birds—or really, parts of dead birds. Other birds went missing. The death of an eight-day-old chick, the first to hatch in the entire nesting colony, hit Greenwell particularly hard. “Every single day, I watched that chick grow,” she says. “The parents were looking for it. They wouldn’t leave the nest, because they were waiting for the chick to come back.”

Meanwhile, residents saw a cat prowling in the area again. It was white. Over the next few days, as birds kept dying or disappearing, wildlife cameras confirmed the presence of the white cat. A resident even snapped a photo from an apartment balcony, in which the cat appeared to be in the sanctuary eating something.

Read: Why we think cats are psychopaths

That’s when they decided on the stakeout. On the night of December 1, Greenwell and the five residents took shifts. The white cat came at 7 p.m.; they shooed it away. The cat returned at midnight; they shooed it away again. The cat came back a third time that night, and Greenwall saw it slinking toward the birds. They chased the cat for half a mile before it disappeared in the coastal scrub. The group returned to stake out the area the next night, and then the City of Mandurah hired an overnight security guard for a few days. When the cat was not seen again, they thought the danger had passed.

But it had not. The cat returned, and more dead birds turned up. What’s more, Greenwell observed, the adults stopped spending time on the ground caring for the chicks. They also stopped working together to drive off raptors hunting for chicks, which adult fairy terns normally do by flying around and above the nests in a big group. The birds seemed to have given up on protecting their chicks. “The colony basically just fell apart,” she says. By mid-December, the colony had left, and all the chicks were dead.

(During this period, two other cats, both identified as pets, were also trapped at or near the site, though there’s no evidence either hunted birds. The fairy tern deaths continued after these cats were removed, too. No other cats were caught on camera or seen by residents at the time.)

“There was absolute outrage in the community,” Greenwell says. The residents had become so invested in the fairy terns that one of them was waking up in the middle of the night to check on the birds. In July, the city council moved forward on a law cracking down on cat ownership. The law would require a permit for more than two cats, prohibit cats from certain nature reserves, and fine owners $200 not following permit rules or for letting their cats cause a nuisance.