Thousands of wild birds' eggs have been discovered at a house in Cleethorpes, the Royal Society for the Protection of Birds (RSPB) has said.
It is illegal to take wild birds' eggs
A spokesman for the RSPB said the haul could be one of the largest in Britain and included eggs from protected species such as ospreys.
Taking such eggs from a nest is a criminal offence which can result in a hefty fine and a six-month jail term.
A 40-year-old man is being questioned by officers from Lincolnshire Police.
RSPB inspectors will examine and catalogue 22 boxes of eggs found at the house in Phelps Road.
Mark Thomas, RSPB investigation officer, said: "It's one of the largest seizures I've been involved with in the last 10 years."
Wild birds' eggs are covered by the Wildlife and Countryside Act 1981
It is an offence to 'intentionally take or destroy the egg of any wild bird'
It is also an offence to possess an egg which has been taken illegally
In 2004, the RSPB received 479 reports of potential offences against wild birds
A Lincolnshire Police spokeswoman said between 5,000 and 10,000 eggs were found.
Since 1954 it has been illegal to remove any wild birds' eggs, and rare birds such as ospreys, peregrine falcons, avocets and golden eagles have extra protection.
Anyone convicted of stealing such eggs could be fined as much as £5,000 for each egg and jailed for up to six months.
The raid in Cleethorpes was carried out jointly by the RSPB and Lincolnshire Police and is part of a national campaign, called Operation Easter, led by Tayside Police.