Advantages of gregarious vs solitary behaviour?
I will assume that you are talking about Mallards, although other wildfowl are likely to exhibit similar behaviors.  

Mallards aren't actually solitary during the summer, instead they form breeding pairs.  They form groups (sords) in the winter to migrate and for protection while moulting.

https://sta.uwi.edu/fst/lifesciences/documents/Anas_platyrhynchos.pdf
http://thefarmatwalnutcreek.com/Ducks_Swans_Mallard-Duck.html