Beached Birds Research Articles

Shetland beached bird surveys: national and European context

SynopsisA Shetland-wide beached bird survey was established in March 1979 and monthly surveys have continued since. This paper presents results from March 1979 to December 1992 during which time a total of 34 320 corpses of seabird and seaduck were found, of which 34023 (99.1%) were specifically identified. Corpses of guillemot (8911), fulmar (8707), kittiwake (4421), herring gull (2264), shag (1843) and great black-backed gull (1785) comprised 82.1% of the identified total, the remainder being made up of 31 other species. The relationship between the numbers of each species found, and their numbers and occurrence in Shetland waters is discussed. A total of 2781 corpses were judged to have been oiled before their deaths, of which 2732 (98.2%) were specifically identified. The six species most frequently found oiled were guillemot (1170 corpses), fulmar (543), kittiwake (184), razorbill (131), puffin (111) and shag (99).Variations in both the percentage of corpses found that were oiled, and the number of oiled corpses found per kilometre of beach surveyed, are examined and discussed. Comparison is made with results from beached bird surveys elsewhere in the British Isles and in other countries bordering the North Sea. In some species, notably the pelagic auks, there was considerable annual variation in the numbers of unoiled corpses found while in other species the number of unoiled corpses found changed more gradually over the 14 years. Reference is made to similar changes observed in other beached bird surveys and possible reasons for these changes are discussed.

Morphologische Variation einer europäischen Population des HaubentauchersPodiceps cristatus in Beziehung zu Alter, Geschlecht und Jahreszeit

A sample of 1450 Great Crested Grebes from the non-breeding season (August–March) which had accidentally drowned in fishing nets in Lake IJsselmeer, The Netherlands, was examined for age and sex differences in linear measurements, body mass, plumage, leg colour and gonad condition. During skinning and drying, wings of non-moulting birds decreased 2.41 mm in length, and wings of wing-moulting birds significantly more: 4.72 mm. The shrinkage is due to changes in the feather follicles and wing elements and not to changes in the actual feather length. Measurements of body, wing, bill, tarsus and keel lengths showed that males are larger than females. The sexual dimorphism is most pronounced in bill length (11 % difference). Differences between juvenile (first winter) and adult birds are small or non-existent. A discriminant function incorporating wing and bill length (age groups lumped), correctly classified the sex of 89 % of the original sample, and 85 % of an independent sample of starved beached birds. The presence (in juveniles) or absence (in adults) of brown lesser upper wing-coverts is a reliable ageing criterion during the whole non-breeding season: only 2 % juvenile females are misidentified. The striped head plumage of juveniles disappears during winter, but there is a large variation between individuals. Adults tend to have one or more greater upper wing-coverts with white inner vanes. Adults often have a wider white stripe between eye and crest than juveniles. Adult males in summer plumage have longer crests and tippets than females. The colour and barring patterns of the inner tarsi vary greatly. Adults generally have a higher density of black bars than juveniles. Adult males appear to have two peaks in testis size: one in spring and a second one in autumn. The grebes weigh least during the wing moult period in August–September. Average body masses of all age and sex groups increase from September to November and then remain essentially stable until April. This pattern is very different from the peaked body mass pattern, described for dabbling ducks and waders wintering at the same latitudes, but resembles the body mass patterns of two sawbill species which also feed on fish in Lake IJsselmeer.

The Shetland Beached Bird Survey, 1979–1986

The results of 7 years of monthly Beached Bird Surveys in Shetland (March 1979 to February 1986) are described and discussed. The occurrence of corpses of the more frequently found species showed highly seasonal patterns which changed little over the period. Densities of Guillemot corpses increased greatly, mainly due to wrecks of unoiled birds during winter. The occurrence of oiled corpses showed a marked seasonal pattern, being highest during winter and lowest in late summer. The incidence of oiling was highest in 1979180 and remained at a lower, fluctuating level in subsequent years. Few oiled corpses were found in the vicinity of the Sullom Voe Terminal since early 1979. Most oil pollution was considered to have originated from offshore sources, particularly to the east of Shetland.

Oil pollution and seabird populations

Large numbers of seabirds may be killed from time to time by individual incidents of oil pollution, and throughout the year, especially in winter, dead seabirds, many of them oiled, are washed up on our shores. These dramatic events have given rise to a great deal of public concern about the effect of oil pollution on the wellbeing of seabird populations. It is important to consider this question from the point of view of population dynamics of seabirds so as to determine whether or not the observed mortality is substantial and additional in relation to the natural mortality. Such an approach requires detailed information on the distribution and numbers of seabirds at breeding colonies and at sea in their pre-breeding years, in association with their breeding activities and also in their ‘wintering’ areas. While the data from breeding colonies provide censuses of breeding birds in defined geographical areas, movements and dispersal of breeding birds result in great uncertainty about their distribution and abundance while at sea. Since many oil polluting incidents, and much of the chronic oil pollution, affect birds while they are at sea, it is very difficult to asess the size and the provenance of the populations of various species that are actually at risk. Some evidence about the numbers of birds killed by oil and other causes can be obtained from both the beached bird survey, which are carried out monthly throughout the winter, and also from the recoveries of ringed birds. These sources of evidence give rather different results, but both are subject to difficulties of interpretation. Most seabirds are long-lived, with low mean annual adult mortality rates, and many of them do not breed until they are several years old. An attempt is made to relate the numbers of birds found dead, and the numbers oiled to the numbers that might be expected to die according to the measured rates of annual mortality. Again there are very considerable problems in attempting to relate these two sets of information. However, it seems that in western European waters the numbers killed by oil pollution is in tens of thousands per winter on average, while the number expected to die naturally is in hundreds of thousands per year. It is not known whether or not oil-induced mortality is additional to natural mortality. It is also pointed out that current environmental circumstances seem favourable and that the present resilience of populations may not persist if conditions change. Emphasis is placed on the very large numbers of pre-breeding birds and the need for information on the means by which they are recruited to breeding colonies. Recent monitoring of the numbers of breeding seabirds throughout Britain shows that most populations are increasing.