All but one of them made 'the migration' in a timely fashion. However, a fortnight later my mum had a 'collect from the sorting office' note from the postman (or post service officer, in these PC times). This was for the last chick. By some accident this particular chick was standing up rather than lying down in the envelope making the package too large for 1st class mail. She had to pay ~£6 for the safe arrival of the last (165th) chick.
Incidentally they were all named and I emailed my father who could be expected on a given day. The first letters of their names obviously made up a 'happy easter' letter slightly longer than a 'tweet'.
This little experiment in novelty gift giving was dependent on those employed to manually sort the mail and their sharp brains. Yep, I deliberately gave them the run around! But it was all worth it. I for one will be sad when such prank-art-projects-via-post are no longer possible because the machines will have fully taken over.