The main cause of the extinction of the pink-headed duck from the forests of India is changes in drainage. Here is a detailed explanation of this answer:

1. Introduction:
- The pink-headed duck (Rhodonessa caryophyllacea) was a species of duck that was once found in the wetlands and marshes of northern India, Bangladesh, and Myanmar.
- It had a distinctive pink head and neck, which gave it its name.
- However, it has not been sighted in the wild since the 1950s, and is now considered extinct.

2. Changes in Drainage:
- One of the main factors contributing to the extinction of the pink-headed duck was changes in drainage patterns in its habitat.
- Wetlands and marshes are crucial habitats for waterfowl like ducks, providing them with food, nesting sites, and protection.
- However, due to human activities such as agriculture, urbanization, and infrastructure development, many wetlands have been drained or converted for other purposes.
- The drainage of wetlands disrupts the natural hydrology of the area, leading to the loss of suitable habitat for waterfowl species like the pink-headed duck.
- The loss of wetlands reduces the availability of food, nesting sites, and shelter for these species, ultimately leading to their decline and extinction.

3. Other Factors:
- While changes in drainage were the main cause, other factors also contributed to the extinction of the pink-headed duck.
- Habitat loss and degradation due to deforestation and afforestation practices also played a role.
- Cold climate or nuclear pollution are not documented as significant factors in the extinction of the pink-headed duck.

4. Conservation Efforts:
- Efforts to conserve the pink-headed duck were limited, as its decline went unnoticed until it was already on the brink of extinction.
- The lack of awareness and understanding about the species and its habitat requirements contributed to the failure to implement effective conservation measures.
- Today, the pink-headed duck serves as a reminder of the importance of preserving wetlands and the need for proactive conservation efforts to protect endangered species.

In conclusion, the main cause of the extinction of the pink-headed duck from the forests of India was changes in drainage patterns, resulting in the loss of suitable wetland habitat for the species. Habitat loss, particularly the drainage of wetlands, disrupts the natural environment and leads to the decline and eventual extinction of species dependent on those habitats.