As you know collector collects different objects, creatures which are going to be extinct but the way he collects is much inappropriate. So i think that as soon he knows about Howard he captured him and put him inyo glass cage.

The only thing collector does is collecting the precious and the things which are special and when collector got to know about Howard the duck he immediately captured him. I agree with mystery

Collector has got information of howord the duck . He collected

Collector Travels across different universe collecting champions he’s interested in

He did the same with Howard