Most published studies deal with specific infections such as parasitic load of selected organs in a chosen species [2] while necropsy studies are few, e.g. studies of acute haemorrhagic gastroenteropathy as a consequence of oiling and cachexia as primary cause of death [3,4]. Some investigations on avian influenza virus have been conducted in wild birds in German waters and an EU-wide systematic monitoring has been in place since 2004. Cases of the highly pathogenic avian influenza virus of subtype H5N1 (HPAIV H5N1), found in aquatic wild birds in 2006 and 2007, were scattered across Germany but concentrated in waters around Rügen, Mecklenburg-Western Pomerania, at Lake Constance, southwest Germany, and in small lakes in the central eastern parts of Germany [5].

Previous investigations conducted on other top predators such as harbour porpoises (Phocoena phocoena) and harbour seals (Phoca vitulina) from the same ecosystem as studied in the present investigation, the German North Sea, showed that animals suffered from parasitic and bacterial infections, particularly those of the respiratory tract [6-8]. Harbour porpoises from the Baltic Sea showed a significantly higher incidence of severe bacterial infections than those from less polluted waters around Greenland [9]. Polychlorinated biphenyl (PCB) concentrations in adult harbour porpoises were ten times lower in the Arctic than in the Baltic and North Seas [10,11]. These investigations showed that systematic health monitoring and toxicological studies can fundamentally contribute to the understanding of the causes of death, potential pathogens exerting pressure on populations and development of diseases in the wildlife population.

One of the most important findings of these investigations was multi-organ infection with M. avium subsp. avium. In the Netherlands, Smit et al. [23] examined 11,664 individuals from 20 different bird species (predominantly non-passerines) for Mycobacterium infection. The bacterium was found in 82 of these individuals (0.7%). Eurasian Buzzard was the species most frequently affected. Oystercatchers and shelducks were the only two species in the study of Smit et al. [23] which were also investigated in our study. One out of 95 oystercatchers (1.1%) and two out of 64 shelducks (3.1%) examined by Smit et al. [23] were infected with Mycobacterium sp. In comparison, 17% of the individuals in our study revealed an infection with Mycobacterium sp. Knowledge about the distribution and population relevance of tuberculosis in seabirds and coastal birds is currently unavailable. Comprehensive studies, including molecular, biological and serological tests, are urgently needed. Compared to M. bovis or M. tuberculosis, M. avium is a zoonotic bacterium with lesser virulence for humans. Usually it causes localised lesions in humans and animals, e. g. restricted to the lung or lymph nodes. Persons handling carcasses and live animals are urged to take appropriate precautions against infection.

Of all countries bordering the North Sea, Belgium has produced by far the largest number of studies of comparable quality regarding pathology of seabirds and coastal birds. Jauniaux et al. [4] conducted histopathological, bacteriological, parasitological and toxicological analyses of seabirds, particularly guillemots (Uria aalge) found dead on beaches from 1992 to 1995. Cachexia, acute haemorrhage of the gastrointestinal tract and oil contamination were the most frequent causes of death. The authors correlated the results with age, sex and origin (pelagic vs. coastal habitat) of the birds. Particularly seabirds living in the pelagic zone were in an advanced state of cachexia mostly, however, in connection with oil contamination. Our data, in contrast, indicate an equally poor nutritional status in coastal and seabirds. Very similar results were obtained by Jauniaux et al. [3], who investigated a total of 133 individuals (guillemot, oystercatcher, kittiwake, razorbill and herring gull) found dead along the Belgian coast.

Of 67 guillemots found dead along the Belgian North Sea shore from 1993 to 1994, 70% showed signs of cachexia in addition to signs of haemorrhagic gastrointestinal disease connected to oil contamination [24]. It is important to note, however, that our study also included a higher percentage of cachectic birds without lesions. Toxicological studies of the birds investigated in Belgium did not reveal increased concentrations of pollutants, which may have illuminated the findings [24].

In comparison to the studies from Belgian waters mentioned above, which showed acute haemorrhagic gastrointestinal disease to be the most frequent lesion, the birds we examined revealed a much wider range of lesions. The proportion of oil-contaminated individuals was in any case considerably lower in our investigation than in Belgian studies [24] possibly influenced by the different bird species composition in which oil contamination may have caused most of the lesions.

Parasites played a minor role in the material investigated. Several other studies were focused only on the occurrence of parasites (e.g. Borgsteede [2]). A parasitological investigation of the gastrointestinal tract in 25 common eiders collected in the Netherlands from 1976 to 1991 [25] showed that they were infected with 5 nematode, 12 trematode, 1 cestode and 1 acanthocephale species. In some animals, up to 100,000 parasites were found. The authors postulated that this may have had a negative effect on the nutritional state. The differences between other investigations and the present study may result from regional or temporal variations. On the basis of the findings presented here, high parasitic infestation may be ruled out as a cause of reduced nutritional status and decreased population size in the birds investigated. A larger sample size from a longer time period should be investigated to confirm this conclusion.

The importance of cachexia is illustrated by the eider mass die-off during the winter of 1999/2000 when approximately 21,000 birds died due to starvation in the Netherlands alone [26]. While the study mentioned comprised only a parasitological investigation, it was rather clear that the mass die-off was caused by the poor body condition of the birds. In this connection, the high percentage of cachectic individuals in our study should be considered an important indicator of a potentially poor nutritional status in many species. One third of the birds of the present study died from infectious diseases, raising the question about potential immunosuppression or high infection pressure. As the infections were accompanied by cachexia in all birds the ultimate cause of death might have been a combination of lesions and starvation; although it is not clear whether initial starvation led to increased vulnerability to infections or vice versa. The only species that showed a comparatively good nutritional status was the red knot. Among all species, most individual red knots were collected after collision with a lighthouse. This may explain the comparably good nutritional and health status compared to the other bird species. However, several of the individuals killed by collision (both in red knot as well as in bar-tailed godwit) showed severe lesions of inner organs as well. It can be assumed that birds already diseased might be more likely to collide with buildings as weakened birds might show reduced flight ability and are probably more likely to collide with structures in strong winds. However, there is no evidence for this assumption. With respect to the comparison of measured and expected body weights it needs to be considered that decomposition of carcasses may have led to underestimation of body weights of the stranded birds. However, we tried to account for this potential bias by exclusively weighing fresh carcasses and rejecting measurements of older carcasses. In the context of some Belgian studies, an attempt was made to demonstrate the connection between cachexia and heavy metals and/or PCB burden in several hundred guillemots collected on beaches over the course of six years: Cachectic individuals showed a higher burden of contaminants in kidney and liver, also of breast muscle in some birds, than individuals in a better nutritional state [27,28]. The higher pollutant burden is considered to be at least an additional stressor facilitating cachexia [27] and/or indicates a transfer or mobilisation of such pollutants during starvation [28]. Pollutant analyses for the birds presented here are unfortunately not yet available.

Indications of avian influenza or other viral infections were not found in the specimens available for our investigation. Highly pathogenic avian influenza virus of subtype H5N1 (HPAIV H5N1) in wild birds was found in 2006 and 2007 but not in birds from the Wadden Sea of Germany [5].