Lost three birds in three weeks

[devo52]

I found the first one a blue orph dead I figure it was a smaller bird in coop so probably smothered so last week one of my Banthams was dead in the corner and went home for lunch and found a silver laced Wyandotte dead none have injuries or showed signs of being sick just dead any ideas on what I need to do I don't want to lose my flock the rest look healthy but so did the ones I lost

 

[seminolewind]

It would help to hear a flock history
How old, where did they come from, if mixed ages, last new ones added, any known symptoms, etc.

 

[devo52]

All are about 4 months all from Tractor supply co with the xcception of the blue orp from local breeder no symptoms orps were added last and they are about 3 months old