I think it helps to try to break down any complex behaviour into smaller simpler behaviours. Could the more complex behaviour come about through very small behaviour changes in already existing behaviours.OK, that much makes sense.
How would they understand something like that?
As far as I understand any of this, a bird's attachment to it's young is purely instinctual. What I'm asking is what could have motivated the creatures to take that first step towards the behaviors that played in the development of those instincts.
I understand the instinct to sit in a safe place, and like I said it makes sense that creatures would want to lay their eggs in a safe place, and it's no stretch to assume that they would develop the habit of using the same places for both things, but IMO none of this helps to explain the development of parental instincts.
That part I get.
Imagine the bird only laid down next to the egg, and only for part of the time. If even some of the body heat was transfered, or if only some protection was provided against cold, that would create some survival advantage. Then the next generations would contain a higher percentage of birds sitting near their eggs. And if some of these birds laid closer than others, and/or for longer periods of time then that would be selected for. And so on until we get what is a quite specific brooding behaviour.
Parental behaviour and instinct can also be broken down into simpler stages and developed slowly over time.