Question about budgies bonding?

A few years ago I owned a few budgies. At one point I had 1 female, 2 males. Perdita (male), Rainbow (female) and Petey (male). Perdita and Rainbow bonded, but one day something happened and we lost Rainbow. Perdita didn't seem to care at all. We got another budgie, female again. Her name was Snowy (I know real original). She and Perdita bonded and they were really close. A few years later Snowy died because of egg binding, after that Perdita became seriously depressed. He had 2 other budgies to keep him company (we had added to our flock by then). I know I this might sound like I'm anthropomorphizing, but it definitely seemed to by depressed. The vet said nothing was physically wrong with him but he ate less, didn't play much and would just sit around. He no longer interacted with the other budgies. He soon passed away. My question is, if he was indeed depressed over Snowy's passing, why did he not care after he lost Rainbow, but became depressed when he lost Snowy? Both he had been "bonded" to when he lost them.