Intruders decapitated three flamingos and strangled a fourth in a grisly attack at Frankfurt’s zoo, officials said Tuesday.

Police said the birds were killed inside their enclosure by intruders who sneaked into the zoo between Monday night and Tuesday morning.

The zoo said three of the birds killed were more than 30 years old. The fourth was a Chilean flamingo, a species native to the South American country.

The zoo counts several flamingos among its menagerie of animals that includes rhinoceros, hippos lions, giraffes and scores of different birds, insects and reptiles.