I've got a flock of (now) about 18 chickens. We've had some deaths and disappearances recently. I have a rotational electro-net setup, 4', with deer netting on the lower 28" to prevent the chickens from going through the charged netting (which they did...and got zapped...and continued to do...)...
Could it be squash vine borers or squash bugs? I lost 6 zukes, about 15 pumpkin vines, 4 butternut squashes to these guys this year. It started the same - some yellowing, some leaves turning brown, low fruit output with discoloration, droopy leaves, then poof! All dead.
Something else to look...