Your birds are showing secondary syptoms of salmonella and other probs due to paratyphoid. it can travel through air on feather dust scales mites etc. keep your birds very warm like a hundre degrees, keep dash of apple cider vinager per gallon for water and chance water whenever see any dust etc in. feed chicken layer pellets, or crumbles if can't pick up pellets, n can gently slide layer pellets down to bottom of left side of throat to force feed gently sort of for birds under weight or not eating well.can also feed kaytee exact formula, that has close to crop milk etc, but slow n messy. i give cheap proios to put in good gut bacteria etc n push. out etc bad ones, n acv will help with as bad bacteria n such generally don't do so good in acidic environment, but good bacteria etc n enzymes do good from. was birds if/when dirty n dusty etc using old fashioned dawn dishsoap as always kills bad bacteria etc shed all over bird from every opening etc. wash oras least scrub down/off everytin dawn as soon as see dirty, especially white n the discharge. maybe invest in cheap heater ni air purification machines and have quarantine shed or room. again keep feed water perches feet feathers faces vents a very clean as are greatest way birds reinfect selves and others, and made easier recovery if on wire bottomed lower cage and just use bricks or blocks for perches. Keep drop area under wire bottom and wire bottom very clean as dust from n poop largest air borne
Spreaders, as well as nasal discharge, and feather dust. spread from flapping, preening each other, sneezing and getting on others and in feed n water, n were walk n perch. birds may develope large swollen or rounded masses on upper legs n or wingsn neck,causing twisting of n or lamness of parts of or whole body, or bumble foot type swelling or sudden syptom of. these will disipate after cause gone and not wryneck etc. seizures may develope butwill disapate, as well, as canker seeming syptoms. birds can carry and not show or easily re show syptoms noticably for years n spread, but sudden cold chill, shipping especially all confined to dirty box etc together, as any decent stress can cause flare up and shedding of disease. by time most would notice outword syptoms its already been spread.
I've saved some birds from culling that then made great fliers and stock birds once immune systems supplemented etc then keep that immunity and are not sick again, as a lot of birds get reacurring and pass bad and not good on.