That is not the implication. In the article they explain that the juvenile cowbirds spontaneously leave their brood host's nests at night and mingle with other cowbirds.

I don’t see how a large group of 4 week old cowbirds can know what’s correct cowbird behaviour anymore than a single 4 week old. It would be the blind leading the blind.

If your assuming they acquire this knowledge on the night of, how do you suppose the cowbird group accomplishes this over a few hours in the field?

And why do they do that and their 'siblings' don't?

What's the purpose behind asking why unrelated animals do unrelated things? What would be a good answer, they're different?

At some point they must realize, "I'm different, these are not my people, I must find others that are like me".