I am playing with only the default space age mods on 2.1, when I try to load only this mod in addition, I get the error:

Failed to load mods: Error while loading entity prototype "chicken" (unit): Product prototype is expected to "item" but nil was given in property tree at ROOT.unit.chicken.loot[0]
Modifications: Animals

Thank you!

replaced data.lua lines 75-81 with this:

    animal.loot = {
        {
            type = "item",
            amount_max = 1,
            amount_min = 1,
            name = name .. "-loot",
        }
    }

which is my best guess after looking up https://lua-api.factorio.com/latest/prototypes/EntityWithHealthPrototype.html#loot

now, Factorio at least start again...

EDIT: but see this, too: https://mods.factorio.com/mod/fruit/discussion/6ae7e144c0fb2fb21878aa0e they overwrite the loot properties, which have to be updated to the new 2.1 syntax as well...