Thieves stole four rare stone curlew eggs from a nesting site in a blow for populations on a nature reserve.
Suffolk Police said the bird eggs were taken from Cavenham Heath, 10km north west of Bury St Edmunds, between May 4 and May 9.
The stone curlew is a crow-sized bird with a large head, long yellow legs and relatively long wings and tail.
It is a rare summer visitor to southern England and East Anglia.
Natural England senior reserve manager Chris Hainsworth said: "The stone curlew is a very rare bird with fewer than 400 pairs in the country and we are shocked and saddened by the theft of these eggs.
"Only 30 years ago, we nearly lost them from the English countryside altogether and it is only by the hard work of local farmers and landowners - supported by Natural England and RSPB - that we have managed to start to see the population slowly rise."
Sgt Brian Calver, of Suffolk Police, said stone curlews and their nesting sites are protected by law.
"Egg collecting is nowhere near as common as it used to be but sadly still goes on," he said. "It's not a victimless crime, as rare populations can be adversely affected by the actions of these individuals.
"The nesting sites are naturally quite remote so we would urge those out and about on such reserves to be vigilant for suspicious activity and report any sightings to police, with detailed descriptions of persons and vehicles.
"If anybody sees somebody stealing eggs they should call 999 as this is a crime in progress."
He said the eggs, which have detailed individual markings, have previously been measured and photographed.
These images have not been released by Suffolk Police.
Sgt Calver also urged people to keep dogs on their leads when around nesting sites.
Anyone with information is asked to call Sgt Calver on 101.