The last thunder

In Pertkwugo, until the death of the thunderbird, this mighty animal served many functions: protector of the city, provider of fresh water, and funerary service.
When someone died in Pertkwugo, their clan's people would prepare them to meet the thunderbird: they would be stripped of their belongings, bathed in the mangrove waters, and then wrapped in a hammock-like shroud. During this process, the whole clan used to gather around and help out, talking about their clansperson. Then, two root-minders would tie themselves to each side of the shroud, and begin the exhausting climb up the great tree in the middle of Pertkwugo, where the thunderbird nests.
Once they reached the nest, the root-minders would tell the Thunderbird about the deceased's life: their great deeds, how many solar cycles had they seen, in how many spawnings had they participated... The thunderbird sometimes would ask further questions, other times they seemed content with the information provided. Once the root-minders left the nest, the thunderbird proceeds to eat the corpse.

What about foreigners?

People who are not part of the clan would only be fed to the thunderbird if they were well known by one of the grippli clans and if it was known that they were ok with this type of funeral.
If the preferences in regards to how to leave this world were unknown for the deceased, and they had no family to take care of it, they would be returned to the sea. They would build a raft, put the corpse in the raft with their belongings, and wait for the right currents to release the raft.

Perception

Gripplis see in this final act as a way of giving back for everything the thunderbird has done for them. By providing the bodies of their deceased to the thunderbird, they provide the thunderbird with a regular source of food. And by telling the stories of the dead to the thunderbird, the thunderbird gets to know those that they protect better.
The nest of Pertkwugo is the most coveted nest in all of Oceasile for a reason. Not only are the inhabitants happy to have a thunderbird around, they also provide the thunderbird with a host of people, the root-minders, to help them in their nest upkeep, to talk with them when the thunderbird feels like having a chat, and they provide them with a constant supply of food.