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The 3 fluffy ones are probably sick. Birds usually fluff up to conserve heat when they're ill. Get them to the vet ASAP and explain your situation to him/her. Birds tend to hide their illnesses until it's too late and die suddenly. Hope you're birds can get some help. Good luck! I've had finches for several years and have seen many different types of illness. It could also be something contagious they died from. Only the vet will know for sure.

The fact that you said you were eating dinner made me wonder if you were in or near the kitchen. Teflon and other non stick cookware when overheated will give off fumes that will kill a bird within minutes of exposure. Also the fumes can come from self cleaning ovens ,toaster ovens, hairdryers, etc. Look into this possibility . The fumes aren't too good for people either.

They could have a disease that they are passing or the fact that you just made dinner could mean that something was used that is making them sick and killing them. Teflon coating is the worst. Also any smoke, any aerosol sprays. These things can be toxic to birds very fast. Teflon fumes that you cannot smell can kill in minutes if not seconds. many things other than pans contain Teflon to. Heaters, self cleaning ovens, fryers, coffee makers. Just fat burning can cause respiratory distress. So sorry for your loss I would take remaining birds to vet better safe than sorry.