The great seabird mystery
This year for Springwatch I'm investigating the great seabird mystery: why are some colonies doing fine while others are in crisis?
With some of the world's biggest and best colonies found on our coastlines, Britain is internationally important for seabirds. 90% of the world's Manx shearwaters, 68% of the world's northern gannets, and 60% of the world's great skuas are found in Great Britain and Ireland.
Although this year things are looking good, Britain's seabirds have been having a hard time recently. Last year, kittiwakes, Arctic terns and Arctic skuas had a terrible breeding season. In the far north of the UK, almost no chicks were reared to fledging. Guillemots, puffins and razorbills also suffered.
Initially, the problem was thought to be caused by their food. Many seabirds feed on sand eels. Although not really exploited for human food, sand eels are used for animal feed and fertilizer so they are a target for industrial fishing.
This fishing, some argued, was affecting seabird levels, especially that of auks because they take sand eels in deeper water. Because of this, fishing for sand eels was banned in the North Sea, and the seabird populations improved.
But when the seabird population subsequently crashed again, it became clear fishing could not be the major cause. Research carried out points to other factors.
The Centre for Ecology and Hydrology carried out studies on the Isle of May. It found links between declining kittiwake productivity and higher local sea temperatures. For sand eel eggs to hatch into larvae in winter, they need the right sort of plankton, at the right time. But higher winter sea temperatures change the composition of the plankton, so the right sort of plankton wasn't available for the sand eels.
The Sir Alister Hardy Foundation For Ocean Science recently announced a staggering 70% reduction in the biomass of zooplankton in the north east Atlantic since the 1960s. On top of this, the cold water zooplankton Calanus finmarchicus is being replaced by a warmer-water species, Calanus helgolandicus.
This is changing the range of animals such as basking sharks, turtles, several species such as sunfish and swordfish and invertebrates such as octopus and squid. It's also possible that climate change is responsible for the population explosion of pipefish in the past five years, which seabirds mistake for palatable prey, resulting in chick deaths and starvation.
According to BirdLife's Climatic Atlas of Breeding Birds, which scopes the impact of various climate scenarios, "five seabird species (which spend some or all of their lives at sea) are projected to 'lose suitable climate' (and thus habitat) and therefore likely to suffer extinction from the UK: Leach's petrel, Arctic and great skuas, red-throated and black-throated diver."
It might also be to do with what's happening to the birds when they are elsewhere in winter.
Herring gull numbers have declined by more than 50% in the last 25 years. The RSPB says the boom in urban gull populations has masked an overall decline in herring gulls nationally. Surveys carried out on the Isle of Man show there was a 28% drop in the number of breeding herring gulls between 1985 and 1999.
Their decline could be linked to the decline of the fishing industry, because traditionally they have fed from the discards from trawlers. Their eggs and young are also vulnerable to foxes and rodents and they may be losing out to other species, particularly the lesser black-backed gull.
Thankfully, there are some success stories. Seabirds on the Farne Islands had a successful breeding season last year. The internationally important population of shags there had their best breeding season for over a decade.
The seabird colonies in Wales have remained largely stable and in Shetland, where I've spent a lot of time lately, numbers of Arctic terns are higher than they have been for many years. Of course we'll have to wait for the results of the breeding season before we know for sure.
Let's hope for some more happy news like this soon. Overall, it's a complex and dynamic problem and one that we should be open-minded about, while looking at all the different influences to try do something positive for seabirds in Britain.
It's also worth remembering that seabirds are long-lived so they only have to produce two young in a lifetime in order to maintain a stable population. So long term these bad years may not be such a problem.
More information about Britain's seabird colonies:
- The National Trust Farne Islands
- Manx shearwaters on Skomer
- Manx shearwaters on Bardsey
- Terns on the Skerries
- Ynys Feurig, Cemlyn Bay and The Skerries
- South Stack Cliffs
- Cemlyn Nature Reserve
- Gannets on Grassholm