Rare seabirds are struggling to breed successfully on a Hampshire reserve, according to the RSPB.
A study by the charity revealed certain gulls and terns at Langstone Harbour, Portsmouth, had "below-average production levels this year".
The lowest level of breeding success was found in little terns.
They built 11 nests this summer but bore no young. The recorded high number of black-headed gulls may have had an affect on them, the RSPB said.
"A lack of suitable nesting sites for all the gulls and terns is a major problem"
A spokesman said: "[The] 4,886 pairs [of black-headed gulls] is also thought to be having an affect on the little tern's success as both species struggle for nesting space on South Binness Island [in Havant].
"Despite this, the report found black-headed and Mediterranean gulls, plus common and sandwich terns also had below average breeding seasons. "
Chris Cockburn, RSPB Langstone Harbour warden, said: "The Little Tern is a declining species across the UK and is high up on our agenda.
"In 1995, around 100 pairs would be seen on our reserve, now we're barely into double figures.
"A lack of suitable nesting sites for all the gulls and terns is a major problem, causing them to unnaturally nest together in high density to the detriment of smaller species."
The little tern is one of the rarest tern in the UK, with only 2,000 breeding pairs a year.
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