Seagull excrement has to be regularly removed from the Spinnaker Tower
Contractors abseiling around Portsmouth's Spinnaker Tower have noticed signs of the local seagull population falling.
While cleaning the exterior of the 170m tower, they have noticed a much smaller amount of seagull droppings.
An RSPB spokesperson said: "Many of the UK's gull species are in decline".
Seagull excrement can react with water to produce a mould which attacks the masonry of the Spinnaker Tower.
Cleaners are called in to clean the viewing windows and exterior structure of the Spinnaker Tower several times a year but recently have only had to spend half the time cleaning the droppings from the tower compared to the same time in 2009.
Herring gull numbers are in decline
RSPB spokesperson Sophie McCallum said: It's very difficult to calculate gull numbers at just one site, but we know the herring gull population is falling dramatically."
The UK breeding population of herring gulls has declined from 750,000 pairs in 1993 to just 378,000 pairs now.
As a result, the herring gull is now classified as on the Red List of species most in need of conservation.
The figures may seem surprising to anyone living on the coast where seagulls nesting on rooftops and foraging in rubbish are seen as vermin.
However the RSPB says more birds moving into town is a symptom of a lack of food in their natural habitats.
Sophie McCallum said: "These are wild birds just trying to survive.
"Their marine habitat and food availability is under threat which is why we see so many of them moving into towns, but the population as a whole is under threat."
Ms McCallum appealed to people not to feed seagulls in towns which can cause them to become a nuisance.