A reduction in bird populations leads to more plant damage from insect consumption and more diseases spread to humans, according to a study published in 2004 in the Proceedings of the National Academy of Sciences. The study predicted 10 percent of all bird species will disappear by 2100.
The study cited lower vulture populations in India in the 1990s as leading to more rabies deaths in the country. Dogs and rats became rabid upon eating decaying flesh because vultures were not consuming as many carcasses. In 1997 alone, India reported 30,000 rabies deaths, accounting for more than half the world's rabies deaths that year.