are nest scavengers, which feed on animal material such as faeces or dead nestlings. Protozoan blood parasites of the genus Haemoproteus have been found in common starlings, but a better known pest is the brilliant scarlet nematode Syngamus trachea. This worm moves from the lungs to the trachea and may cause its host to suffocate. In Britain, the rook and the common starling are the most infested wild birds. Other recorded internal parasites include the spiny-headed worm Prosthorhynchus transverses. Common starlings may contract avian tuberculosis, avian malaria and retrovirus-induced lymphomas. Captive starlings often accumulate excess iron in the liver, a condition that can be prevented by adding black tea-leaves to the food. Distribution and habitat The global population of common starlings was estimated to be 310 million individuals in 2004, occupying a total area of . Widespread throughout the Northern Hemisphere, the bird is native to Eurasia and is found throughout Europe, northern Africa (from Morocco to Egypt), India (mainly in the north but regularly extending farther south and extending into the Maldives) Nepal, the Middle East including Israel, Syria, Iran, and Iraq, and northwestern China. Common starlings in the south and west of Europe and south of latitude 40°N are mainly resident, although other populations migrate from regions where the winter is harsh, the ground frozen and food scarce. Large numbers of birds from northern Europe, Russia and Ukraine migrate south westwards or south eastwards. In the autumn, when immigrants are arriving from eastern Europe, many of Britain's common starlings are setting off for Iberia and North Africa. Other groups of birds are in passage across the country and the pathways of these different streams of bird may cross. Of the 15,000 birds ringed as nestlings in Merseyside, England, individuals have been recovered at various times of year as far afield as Norway, Sweden, Finland, Russia, Ukraine, Poland, Germany and the Low Countries. Small numbers of common starlings have sporadically been observed in Japan and Hong Kong but it is unclear whence these birds originated. In North America, northern populations have developed a migration pattern, vacating much of Canada in winter. Birds in the east of the country move southwards, and those from farther west winter in the southwest of the US. Common starlings prefer urban or suburban areas where artificial structures and trees provide adequate nesting and roosting sites. Reedbeds are also favoured for roosting and the birds