cerebral hemisphere

But if a bird only deployed this narrow attention, its survival fitness would be low, as every time it focused on finding food, it would be oblivious to its surroundings and easy prey for predators.

To solve this problem, a bird simultaneously deploys a more open, broad, and sustained attention to monitor its surroundings and to be on the lookout for potential threats.

The simultaneous deployment of two types of attention was made possible by the evolution of the two brain hemispheres.

Or as McGilchrist writes, How on earth can you dispose your consciousness towards the world in two conflicting ways at once? The answer is the evolution of two neuronal masses, separate enough to function independently, but connected enough to work in concert with one another, each capable of sustaining consciousness on its own.

In other words, a bipartite brain.