Why did the dodo and solitaires lose the ability to fly?

The dodo and solitaires lost the ability to fly as an evolutionary adaptation to living in isolated, predator-free environments on the Mascarene Islands.1

In the absence of natural land predators, the energy-intensive requirement of flight became unnecessary for survival. Over time, natural selection favored other traits, such as increased body size, while the birds' wings became rudimentary and their sternums lost the large keel necessary for attaching powerful flight muscles.231

While this flightless state was a successful adaptation in their original, secure habitats, it left these birds highly vulnerable when humans and introduced animals—such as pigs, rats, and dogs—arrived on the islands. Because they lacked the ability to fly away from these new threats and were often unaware of the danger posed by predators, they were unable to adapt to the sudden change in their environment, ultimately leading to their extinction.124