Is Acid Rain Killing Off Wood Thrushes?

Acid rain may be forgotten, but it is not gone, and now ornithologists have linked it to the decline of the wood thrush, a forest bird known for its beautiful song.

The wood thrush breeds in the eastern United States and southeastern Canada, and winters from central Mexico to Panama. Like many neotropical migrants, its population has been declining – nearly 2 per cent a year between

1966 and 2000, according to Breeding Bird Survey (BBS) data compiled by U.S. and Canadian wildlife agencies.

The researchers did a statistical analysis and found that increased amounts of acid rain make wood thrushes less likely to breed. Ecologists were surprised to uncover a negative effect this big because like many people, they thought acid rain was a non-issue.

Despite rising awareness of acid rain's more insidious effects, such as calcium depletion in soil, research on declining populations of North American birds has continued to focus on the dangers of habitat loss and fragmentation. Indeed, it is the first time in North America when the finding has linked large-scale evidence of the population decline of a land bird to acid rain.

The situation is reversed in Europe, where, perhaps owing to centuries of habitat fragmentation, researchers have been looking at more immediate threats: industrialized farming practices and pollution, including acid rain. Their studies of calcium depletion raised a number of red flags that may apply to the wood thrush.

Acid rain, for example, can cause calcium to leach from the soil. The loss of this nutrient jeopardizes the breeding success of birds – to produce a clutch of eggs, a female bird may require up to 15 times more calcium than a pregnant mammal of equivalent size.

In areas where acid rain is most severe, the supplementary calcium-rich foods that female songbirds depend on – snail shells, isopods such as pill bugs, millipedes, and earthworms – may be in short supply. Lacking adequate calcium in their diet, females are more likely to lay eggs that are thin, brittle, and porous. If the weakened eggs can withstand the rigors of incubation, the parent birds will be hard-pressed to meet the very high calcium requirements of their growing nestlings.

Aside from depleting calcium, acid rain in soil can promote increased levels of potentially toxic aluminium, cadmium, and lead. Polluted soil, moreover, may slow the decomposition of leaf litter, which reduces the diversity and abundance of prey.

Now it is crucial for scientists to understand the process that leads to these effects. The findings regarding the wood thrush are correlational. To get down to exactly what is going on more focused studies are needed.

Robert Winkler. National Geographic News. 2002

India's Black Market in Birds Threatening Rare Species

India is home to some 1,200 different species of birds. Despite measures designed to protect this rich array of bird life by banning the capture and trade of wild birds, records indicate that as many as 300 of these species are caught and

traded with impunity. Large seizures of illegally captured wild birds in India number about 30 each year. In one of the largest recent incidents illustrating the huge scale of this flourishing black market, more than 10,000 birds were confiscated at the Mumbai (formerly called Bombay) international airport in March 2001.

Until 1991, India was one of the largest exporters of wild birds to international bird markets. Most of the birds traded were parakeets and munias, especially the rose-ringed parakeet, the black-headed munia and the red munia. In 1991, however, an amendment to the Indian Wildlife Protection Act of 1972 was adopted that bans all trade and trapping of indigenous birds in the country.

However, the ban is ineffective, as illegal bird trade flourishes in almost all cities, towns, and rural hamlets of the country. The trade is not limited to domestic markets. There is continuous large-scale bird smuggling out of the country. A number of the wild birds on the Indian subcontinent that are being widely traded are threatened species, including the swamp francolin, green munia, Finnâs baya, and Shaheen falcon. Of the many different species being traded, 16 are among the world's most highly endangered, 36 are listed in the Convention on International Trade in Endangered Species (CITES), and six are included on the Red Data list of endangered species compiled by the International Union for the Conservation of Nature and Natural Resources (IUCN).

Wild birds are captured for at least seven reasons, which helps explain why the practice is so prevalent and difficult to halt.

Besides the usual practices of trapping birds as pets and for food, zoological and medicinal purposes, or taxidermy, several species of birds that are poor candidates as pets and for consumption are often captured for the bird-release business.

This is a uniquely Indian religious tradition. Among Hindus, Jains, and a few other communities, there is a belief that releasing birds that are held in captivity can purify the soul and relieve personal sins. On auspicious days, people go and buy birds from traders for release. This has led to the development of an entire business around the religious custom. Species of wild birds that are unsuitable as pets or food are captured and brought near these holy places for the devout to purchase and re-release.

Some important protected species like the horned owl were hunted for black magic rituals and sorcery, such as the practice of certain tribes that use the owls to purify "amulets" during their street performances.

Finally, although the sport of falconry is now a vanishing art in India, a large number of wild raptors are caught every year to smuggle to the Middle East, where falconry is still popular.

Accurate figures on the capture of wild birds for local consumption are impossible to obtain because of the secretive nature of the practice. It is known, however, that species ranging from pelican to ducks and waders are caught for

consumption. Added to these huge losses is a large number of bird deaths during international transit. For every bird that reaches its final destination, two die en route. The mortality of wild birds during transit is thought to have increased since the trade ban was put into effect because of the extra efforts that are made to conceal the illicit cargo.

Pallava Bagla. National Geographic News. 2002