“Oiled birds” have been spotted around Poole Harbour in Dorset after a pipeline from a nearby oil field leaked on Sunday, the RSPB said.
A major incident was declared after 200 barrels of reservoir fluid – consisting of 85% water and 15% oil – spilt from Perenco’s Wytch Farm oil field into the water.
Poole Harbour is recognised as an internationally important wetland and is a specially protected area as well as a site of special scientific interest.
The RSPB said it is too early to fully assess the damage to wildlife although it is “very concerned” about the effect on migrating birds.
“The area is home to numerous bird species such as black-headed gull, Mediterranean gull, redshank and oystercatcher which breed on the coastline, and many other waterbirds which rely on the harbour mudflats for food.
“There are wintering avocet, black-tailed godwit and curlew still present in the harbour. There may also be a risk to the many birds that fish in the harbour, such as sandwich terns and ospreys.”
Anyone who has come into contact with the spill has been asked to wash immediately with soap and water.
Paul Morton, of the charity Birds of Poole Harbour, said: “Right now we’re in that important transition period where our winter birds are leaving and our summer birds are arriving, many of which use the southern shore of the harbour for feeding, nesting and hunting.
“Each month we conduct wetland bird surveys across the harbour to monitor populations of over-wintering birds. The last survey which was conducted on March 5 logged 5,450 birds along the southern shore in the spill area.”
Poole Harbour is home to the only pair of ospreys in southern Britain, having bred for the first time there after an absence of 200 years, Mr Morton said.
The male returned from West Africa on the same day as the slick while the female is expected in early April.
Mr Morton said: “The elation of the male’s safe return home yesterday morning was soon dashed by late afternoon when news of the oil spill was announced.
“Ospreys exclusively eat fish, and our pair hunt in the harbour about 95% of the time during the spring and summer season, feeding on grey mullet, flounder and bass.
“Right now huge numbers of birds are on the move, having over-wintered further south, and will soon be using Poole Harbour as a service station to feed up before continuing north.
“When it comes to an issue like this there are several areas of concern. One is that any birds that come into physical contact with this substance can see their feathers damaged and become less water resistant.
“The substance will also encourage any affected birds to preen constantly, meaning they could ingest the hazardous liquid.
“The other unknown is how much of the oil will settle on the mud and harbour shorelines and what impact that will have on marine invertebrates.”
Brian Bleese, chief executive of Dorset Wildlife Trust, said: “The impact on other marine species is also of great concern.
“Poole Harbour is a bass nursery and an important area for molluscs which feed by filtering seawater and may be particularly badly affected, as well as resident seals and two species of seahorse that breed in the harbour.
“Our wardens are closely monitoring the Brownsea lagoon where common and sandwich terns are gathering to nest and the northern shoreline to assess any impacts on wildlife there.
“We are liaising with Poole Harbour Commission on appropriate actions that need to be taken over the coming days and weeks.”