Little birdies usually arrive together and leave together - so a half-empty nest doesn't exist for birds.
They do for us humans, though, and throw up interesting situations.
I remember that it was actually tough for me to interact with my parents when it was just me, when R. and B. left home for college. It seemed strange ; took some getting used to. I think our younger one may be feeling the same.
The older siblings usually have more responsiblity, and household chores, and when they go, its not always easy for the younger one to fill those shoes. So in our house, now, we often don't have cold water in the frig. D. used to do that, a lot of the time, and we are still getting used to taking over her responsibility. And I realise she liked cleaning up - now, things she used to put away lie around a bit, and we have to work out who will take over.
And the silence, sometimes, is deafening. I'm so aware of it all the time. Earlier, the two of them were always talking, and the chatter and laughter was comfortable background noise. Now I even miss the occasional raised-voice arguments that would erupt.
I try to fill the gaps for the younger one - but I realise I can't. I cannot step into my daughter's shoes either..... ;-)
