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).
Diamond-shaped boxes with just the right-sized holes attract House Wrens.
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.
I assume this is a female because she still has a bare abdominal area where her brood patch was. First she sat in a tree and scolded me.
Then she sat on a tomato cage and scolded me, quite loudly considering that she kept her beak closed around that caterpillar.
Then she moved to within about 6 feet of the nest box, but still hesitated to enter. That caterpillar is starting to look dehydrated.
Oops, what happened to the caterpillar — did she drop it or eat it? She is sitting right opposite the nest box and giving me a piece of her mind.
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.
“I have something to feed my babies — don’t look while I approach.” This wren made two short hops in a bush next to the nest box and then popped inside with a big, juicy fly.
In and out, task completed — off to find another meal.
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.