Diseased Bird: sick/lethargic finch

I found her perched on the edge of our bird feeder seed catcher. She was completely dazed. Eyes closed. She wasn’t even aware I was mere inches from her. I grabbed her with my hand. She struggled some which gave me hope. I placed her in one of our clean transport cages. Within about 30 minutes, she passed. No visible/physical sign of injury or disease but something was clearly wrong as she exhibited labored breathing.