It started as a well-meaning effort by Google technology workers to look after feral cats living around their headquarters in Silicon Valley.

But now the felines are suspected of killing rare owls in a nearby wildlife refuge, leading environmentalists to condemn the cat rescue effort. A handful of burrowing owls make their nests in the long grass in Shoreline Park, a 750-acre expanse next to Google's massive offices at Mountain View, California south of San Francisco.

The cats are thought to have been released by an employee group at the company called GCat Rescue.

They set out to trap feral cats, putting captured kittens and tame adults up for adoption. More aggressive adults are neutered and released with tacking chips.

Feeding stations were then set up near the offices, a process the group describes as “colony care".