Eggs from black-headed and Mediterranean gulls in most of the 9,000 nests on the harbour's remote islands were taken in May 2016.
Charity Birds of Poole Harbour said the eggs -  considered a delicacy - were likely to used by  restaurants.
The nesting gulls are being monitored by Dorset Police's marine unit.
The force said the birds would be checked day and night by officers through the breeding season.
Joel Brooks, of the force, said: "The collection of birds eggs is illegal unless licensed and no one is licensed to collect in Dorset.
"We have wildlife officers from the Metropolitan Police in London making enquiries and checks on the establishments likely to be buying and selling the eggs."
Birds of Poole Harbour said there are around 20 "pickers", operating under licenses issued by Natural England, which allows black-headed gull eggs to be collected legally at five sites in England.
Restaurants or pubs have to be shown a valid licence before buying eggs to prepare in meals.
Paul Morton, who runs the charity, said: "It's been proven black-headed gull eggs are safe to eat - but there has never been a test done by Defra on Mediterranean gull eggs to confirm these are safe for human consumption."
The charity said it planned to survey the islands again later in the spring to see what effect last year's thefts has had on the population of the two species of gulls.
Police patrols are being carried out on islands off Poole Harbour to protect eggs from protected bird species after hundreds were stolen last year.