A good option! At the very least, I'd alert the person you got them from that the birds developed these symptoms.... and if you do start over, I'd consider going somewhere else to get them.
Still, though, I'd consider getting a test. Mycoplasma can hang around in the soil and infect new birds...
You did say they were your only birds. I missed that in your post. Sorry!
Not every antibiotic treats every infection... that's why it's important to know what kind of illness you're dealing with.
For mycoplasma, TerraVet water-soluble powder treats it. :)
Let us know if you can get a...
Did you quarantine those birds before adding them to your flock? Respiratory illnesses can be highly contagious.
Your course of action and treatment depends on what is causing the illness.
If it's mycoplasma, the birds, if they survive, will be carriers for life and can infect other birds...