Rare birds poisoning toll reaches 16
3 April 2014 Last updated at 15:56 BST
Four buzzards and 12 red kites have been found dead in one area in the Highlands in a few weeks.
Bird experts say it is the worst poisoning incident of its type in Scotland.
Ch Insp Ross MacWilliams, of Police Scotland, said: "We will be following up every possible line of inquiry to identify the person responsible and make sure they face the consequences of their actions".
Police Scotland have urged anyone who finds a dead bird to contact them and not to touch the birds because of the danger of the poison.
BBC Scotland's Craig Anderson reports from the Black Isle.