Birds build nests primarily to provide a safe and suitable environment for incubating their eggs and raising their chicks. Nests help maintain the correct temperature and humidity needed for egg development and protect the eggs and young birds from predators and harsh environmental elements such as wind, rain, and extreme temperatures. Additionally, nests serve as a secure place for parents to care for their offspring after hatching.

Nest building is not purely instinctive; some bird species learn nest-building behaviors from others, resulting in variations and sometimes distinct "cultural" nest styles within groups of the same species. However, the fundamental reason remains the protection and successful rearing of their young.

Birds choose nesting sites that offer safety from predators, shelter from the weather, proximity to food sources, and sometimes the presence of safe roosting spots near the nest. Different species use various materials and build different styles of nests depending on climate and environmental conditions to optimize warmth and protection.

In summary, birds build nests mainly to incubate eggs safely and rear their young by providing warmth, protection, and shelter. This behavior may mix instinct with learned elements in some species.