Duckling looking swollen...?

If the swelling is just in the crop area, it may be a full crop, but if it is all around the neck it could be a leaking air sac, or subcutaneous emphysema from a possible injury. It is kind of like a collapsed lung in people. If the duck appears in distress, it can be deflated using an 18 gauge needle under the skin and pushing the air out. If you think that it is from bee stings, then you could use 1/2 of a Benadryl tablet orally. Here is a link to read: http://ijasvm.com/ijasvmadmin/upload/IJASVM_5113cbc51d31b.pdf

 

Crops can be full during the day, and then should empty by morning after not eating at night. If the crop is big and hard, it could be impacted with grass or other things that won't go down, or big and balloon-like and squishy, which is called sour crop, and usually do to long-standing impacted crop. You may find better advice on the duck thread here: https://www.backyardchickens.com/forums/threads/add/forumId/42, but here is a crop link about chickens: http://www.the-chicken-chick.com/2012/04/answers-from-chicken-vet-on-impacted.html