Both examples, the unrequited loves and the familial problems, pertain to responsibility, which the son mentions as he leaves. While his example about the birds is odd, he may as well be referring to their lifelong irresponsibility, especially the man's, which we still witness. They regret the past, not taking responsibility for the path they have chosen, and they have to craft a fantasy- present in order to escape their real lives, another irresponsible gesture. As stated previously, the existentialist believe that only a responsible, committed life could be meaningful, and it looks more likely that the man's message will be his last-ditch effort to gain such meaning.
It is a remarkable achievement that Ionesco can create a palpable sense of excitement for the man's message as the rooms fills—with invisible people. Obviously, the chairs help set the mood, but the incessant action creates the sense of chaos and mass in the room. Ionesco calls his play a "tragic farce," and this section, above all, lives up to that billing. While the characters' scrambling to seat invisible guests on chairs is comical, the illusion is nonetheless poignant, and even disturbing.