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Migration takes up a lot of energy, so I am wondering if birds (in my case mallards) change their diet / food preferences in the weeks before migrating. Do they select for more energetic foods to fuel up, or do they just eat more of the same foods to gain enough energy?

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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.

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Geese can build a lot of fat reserves before migration, it's the reason that Fois-Gras is made, because the goose can keep lots of fat in it's liver.

You can buy Spanish foisgras which is made naturally by letting the geese fatten on nuts and rich autumn foods.

Birds generally eat whatever they can get, as rich a diet as possible in protein and carbs, but it's dependent on seasons. So the diet changes anyway at that season, independently from the birds energy requirements. https://sora.unm.edu/sites/default/files/journals/auk/v098n01/p0065-p0079.pdf

"The natural cycle for the wild goose of Europe is to spend the summer in Denmark, Sweden, Norway, northern Germany or parts of Russia and Ukraine. Then they migrate south to Africa each autumn," Labourdette explains. "They stop here in Spain on their way, to eat and gain energy for the long flight. But lots of them never leave because they find such a good habitat here."

Geese are adaptable, he says, and they eat whatever the environment provides. Foie gras from Denmark has a fishy taste, because the geese eat seafood there. Here in Spain, they feed on calorie-rich acorns, olives, figs and seeds.

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