a. Increased number of predatory birds.

b. Overexploitation by humans.

c. Non-availability of the food.

d. Bird flu virus infection.

Answer: b. Overexploitation by humans

Explanation: The main reasons for the extinction of passenger pigeons were the high rate of hunting, the loss of habitat, and the extreme bird life, which made it extremely endangered. Deforestation was driven by the need to liberate agricultural land and expand cities, but also because of the need for timber and fuel. Although there are still large forest areas in eastern North America, which support a wide variety of wildlife, it is not enough to support the large number of passenger pigeons needed to feed humans. When herds were declining in size, the number of dove riders dropped below the threshold needed to distribute the species.