Despite what we might think, birds do have personalities, and can be quite different from one another even within the same species. Take the case of two House Wrens who happen to be nesting quite close to one another in cute little wren boxes in this lovely garden (of a friend).
My presence (along with that of a friendly cat) immediately set the wrens to chattering and scolding. I could see that they were busy feeding their broods in the nest box, so I stood quietly and watched.
One wren was quite perturbed by my presence and flew from perch to perch, trying to decide whether it was safe to enter her nest box or not. She had a juicy caterpillar to feed her chicks.
Contrast this behavior with that of another wren (couldn’t tell if it was male or female) feeding its offspring in the box next door.
You can see how natural selection might act here, depending on the type of threat. Wren A’s chicks might not get fed as well and perhaps not all of them would fledge but she doesn’t give away the location of the nest. Wren B’s chicks might get lots of food, but could be easily discovered by a predator, since the bird made no attempt to disguise where it was headed with its prey. Good thing there is such variation in behavior among individuals, insuring that some will exhibit the appropriate response and survive.