Common Cuckoo chick ejects eggs of Reed Warbler out of the nest.

I'm no specialist but calling it genetically encoded, instinct or something else doesn't change the fact that's one specific action to that species of bird. I'm not saying such incidents don't happen elsewhere but for that kind of bird, it does seem to come "naturally".

In the Brow-headed Cowbird, they found one detail about the bird that could explain the parasitic laying of the egg. It may be the case for the common Cuckoo too, but for the Cowbird, both genders lack the Brood (or Incubation) patch. It's an area that develops on the lower abdomen of birds in which the feathers drop off and the skin thickens and becomes densely populated with blood vessels. It's used in incubation to keep eggs and young warm.

Since both genders lack that patch, it means they can't effectively incubate an egg, so they have to let others do it. The theory is that since at the beginning this bird was following Bison herds and eating insects of them, it couldn't build a nest in one place and travel the long distances for food, has the herds moved. So over the years they "lost" that patch. So it's a parasitic way of reproduction but it's not really based on the bird being a pure a-hole, it just can't incubate it's own egg.

Anyway, the dilemma still remains, do you save the nest that has been invaded by sacrificing the egg of a bird who has no other means or do you let an innocent bird family go to wasted by letting it slide?

Even if they are parasites, somehow I doubt they were ever in such numbers that other species went extinct solely on that basis.

Nature is cruel.

We'll I'm no expert myself, I'm just an amateur ornithologist who happens to read on what I see.