The primary cause of the decline is thought to be excessive hunting on the Mediterranean wintering grounds. Habitat loss, particularly in the wintering grounds, may also have played a part, but huge areas of forest bogs suitable for breeding still exist in Siberia. It is unknown to what extent the birds still reproduce successfully, and how much gene flow still exists in what may once have been a large and widely dispersed population undergoing little purging of deleterious recessive alleles and consequently with a high MVP. Furthermore, there is evidence that birds in winter quarters were more numerous once, and in general not a very rare sight in Western Europe in the nineteenth century, where they were hunted with some regularity. Later on they were additionally threatened by pollution, e.g. oil spills. There are no data about how these threats endanger the species today. Theoretically, they might have retreated to all but inaccessible areas, but then, a single hunter or fox might unwittingly wipe out enough of the few remaining birds to doom the species.