I just need a place to talk about it where my in-laws definitely won't find out about it. Thanks for your patience, everyone.
Her name was Tina. She was a blue Pacific parrotlet, and she was about seven months old. She was a good, sweet bird, prone to trying to yank out my cockatiel's crest (it was wrong, see, because she didn't have one, so clearly birds weren't supposed to have one), and generally inclined to try to fix everything and everyone.
My mother-in-law apparently decided it was a good idea to put Tina on my partner's shoulder while she was asleep and then leave the room. I'm not sure where I was. I might have also been sound asleep, or I might have been in the kitchen changing the birds' water and making their daily serving of eggs and vegetables. My partner, being sound asleep, and thinking she'd only dreamed her mother putting the bird on her, rolled over at some point, and from there it seems likeliest that Tina didn't manage to respond in time to avoid suffocation. We wrapped her up in some black cloth, and I buried her next to a flowering plant in the backyard.
This is basic bird safety. You do not put any bird smaller than a Goffin cockatoo on a sleeping person without close supervision. Because my mother-in-law did this, my partner is grieving, my cockatiel will be grieving once she figures out Tina isn't coming back (they were cage-mates), and our parakeet will probably be grieving along with her. I cannot fucking believe this shit.
We're not telling my partner's mother what happened, because this is a bit of a stressful time, and my partner doesn't want to make a fuss. We're in the middle of trying to persuade her that we don't have the resources to properly care for all our own birds (a rescued African Grey, a rescued green cheek conure, a rescued parakeet, a cockatiel, and a Goffin cockatoo we've had from infancy) *and* the neurotic middle-aged Goffin she insisted we rescue ourselves, and we're concerned she might take Tina's death as an indication that the resources to care for him are now free (which they aren't -- cockatoos and parrotlets have very different needs). If she asks, we're just going to tell her we don't know where Tina is.