I have almost the whole flock who suffers from a dark red throat and the larynx is all the time pumping in and out I mean it's never stable?

I have sick birds in my loft whereby they lose weight and there seems to be a sound of slime in the back of the throat?

Hoe komt het dat ik tijdens de vlucht altijd last heb van koppen? Wat kan ik hiertegen doen?

My pigeons have little endurance and return late. What can I do?