Conservationists have called for restrictions on pet flea treatments after research found songbird feathers widely contaminated with substances that can damage the birds’ brains and kill unborn chicks.

Almost every feather sample tested from five common species of UK garden birds contained either permethrin, imidacloprid or fipronil – all insecticides that are banned for agricultural use but still common in pet tick and flea treatments.

Fipronil and imidacloprid are known to impair breeding success in garden birds and to disrupt their neurological function, the researchers said, while permethrin has been found to slow the growth rates and repress the feather growth of wild bird chicks.