Do birds change their diet before migration?
Apparently, yes. This article underlines the changes in diet in semipalmated sandpiper before its 3000 km-long migration:


  Near the beginning of their journey, sandpipers stop at the Bay of Fundy on Canada's eastern coast to gorge on mud shrimp, 1-cm-long crustaceans loaded with omega-3 fatty acids. Over 2 weeks, the frantic feeding doubles each sandpiper's body mass [and i]t makes sandpipers' muscles use oxygen more efficiently, enhancing the birds' endurance.


The authors experimented the diet on a similar species to disentangle the effects of the different diet from other effects, for example due to hormonal changes:


  To isolate diet's role, Weber and colleagues took exercise and migration out of the equation. They turned to the bobwhite quail, an unrelated sedentary bird that doesn't migrate and seldom flies. For 6 weeks, the scientists fed 40 couch-potato quails a combination of omega-3 fatty acids from fish oil. To the researchers' surprise, the quail's oxidative capacity - their muscles' efficiency at using fuel - shot up 58% to 90%.


This paper reviews the effects of diet changes on the digestive trait of migratory birds; however, it's quite old (2001).

I couldn't find any indication on mallard feeding prior to migration, but these links can be a start for further research.