Wednesday, 19 February 2014

Seabird Wrecks

From time to time abnormal numbers of dead or dying seabirds are washed ashore. Such events get referred to as ‘Wrecks’, particularly when adverse weather rather than pollution is the likely cause. After the prolonged series of severe gales experienced on western coasts since mid December, more than the usual numbers of seabirds may be expected to be recorded on Welsh beaches during the annual Beached Bird Survey organised by the RSPB. This exercise was started a couple of decades ago primarily to monitor chronic levels of oil pollution that were then often coming from tank washing or still leaking from the bunkers of vessels sunk in wartime. Nowadays, except when there has been an accident, it is unusual to find oiled seabirds. However, those of us old enough to remember walking beaches in the 1950s may recall that it was unusual not to encounter the remains of oiled auks.

Razorbill wreck
The 2014 Beached Bird Survey now underway is centred on 22-23 February, but records a week either side of this weekend will be included. The survey simply involves walking a length of shoreline between grid referenced locations and recording the numbers of each species that may be found, noting if any happen to be oiled and checking for rings. By pooling data from many beaches in a region it is then possible to derive comparable figures for numbers per kilometre of shoreline.

As might be expected, the numbers washing up on particular coasts vary somewhat from year to year, not just to changing mortality rates in the near shore waters but also on wind directions prevailing in the weeks prior to the count. This year there are already indications that the numbers of Razorbills and Guillemots are going to be higher than normal. The weather is the likely cause, with gales reducing food availability and extra stress while trying to avoid being carried too far down wind. Often in such wrecks the birds are found with flight muscles wasted away as well as having burned up their fat reserves. For the two auks (Razorbill and Guillemot) there are records going back to the 19th century of occasional early autumn wrecks as well as mid winter ones. In the Irish Sea auk wreck of 1969, the primary cause of fatal stress was shown to be problems during the post-breeding moult. After breeding these auks become flightless for at least six weeks while growing a new set of flight feathers. Other species vulnerable to winter wrecks round the British Isles have been Kittiwake (Cardigan Bay, Feb 1959) and Shag.

If early indications from personal observations are borne out by the full survey it seems likely that the species most affected in the waters from which birds have drifted to Anglesey this time is Razorbill. Taking estimates of breeding populations in the British Isles as a rough approximation of expected ratios if Guillemots and Razorbills were equally vulnerable, the expected ratio in body count would be 5 Guillemot to 1 Razorbill. From just the small and statistically inadequate sample I have seen so far the ratio here is reversed. It will be interesting to see if this is borne out when more data is available.

While the number of birds involved may be relatively large, past indications are that this sort of winter mortality will not have much effect on the auk breeding populations in the longer term. Both Razorbill and Guillemot are quite long lived, they do not breed until several years old and there tends to be a pool of non-breeders waiting to take places on the better ledges.

This article posted by Ivor Rees.

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