Starting in the 1980s, an anomaly has occurred in the form of a reduction worldwide in the number of migratory birds. In Japan, for example, summer birds such as the Black Paradise Flycatcher, the Brown Hawk Owl, the Ashy Minivet, and the Ruddy Kingfisher have virtually disappeared.
Source: Hiroshi Uchida / Photographs: Kanehiro Warita
The first thing that comes to mind as a possible cause is deforestation. Not only the cutting down of trees, but the "division" and reduction of forests as a result of the building of roads are also major problems. Some species are unable to survive unless there is a large area of forest, and in some cases this "division" is leading to a reduction in available habitats. However, in the case of Japan's summer birds, many live in protected areas, so it's unlikely that the division of forests alone is the cause of this reduction.
Another possible reason is the contamination of the natural environment with chemical substances. One likely cause is crop-dusting, which has a harmful effect on ecosystems by eliminating the insects that migratory birds feed on, making it difficult for them to raise their young.
Another major problem is environmental change in wintering places. The deforestation of tropical rainforests has become conspicuous in recent years, with over 70% of the tropical rainforests in Indonesia, for example, having disappeared since the 1980s. It's possible that a major cause of the reduction in the number of migratory birds is the disappearance of places for them to rest, not only in wintering places, but throughout the various countries along their migratory routes.
It's also thought that global warming is starting to have an effect on migratory bird populations. For example, the date on which the Violet-backed Starling, a summer bird that visits Japan after wintering in the Philippines, lays its first egg has moved forward a total of around two weeks over the last 20 years. By nature, the laying of the eggs was timed so that the hatching of the chicks, who need to eat plenty of food straight away, coincided with the ripening of fruit in the area, but this timing is now out of synch. As well, it's been observed that the wintering places of winter birds are steadily moving north.
Investigating the ecology of migratory birds, which live in harmony with the environment on a global scale, and finding ways of protecting them will no doubt become increasingly important themes in the future as we consider how to save the environment of the Earth as a whole.