@JadeComputerGal could you help me understand borderline?
It's hotter than I'd want to have a brooder for a 1.5 month old duckling if the duckling were being kept in a brooder. I wouldn't be supplying supplemental heat at that point, which would mean around 75 F. Sounds like that's not the case here since a cage would mean something fairly open with good airflow. That being said, there are many people in very hot climates who raise their ducklings/ducks completely outdoors. I'm thinking this one is either entirely or almost entirely nutritional, though there can be a genetic predisposition to wing disorders like you pointed out. I'm very glad he caught this one when he did before the duckling got its adult plumage.