Softbills, broadly speaking, are categorised as those birds whose beaks are not suitably adapted for cracking seeds, or penetrating the tough outer layers of certain husks and grains. This is in a relatively stark contrast to “hardbills”, which are those birds whose beaks are hard enough to crack open seeds (finches).
However, as with many things in nature, the distinction between softbills and hardbills is not quite so black and white. Robins, for instance, do have the ability to consume and digest very small seeds, but they wouldn’t necessarily crack them open. This is left to those birds with strengthened bills, such as finches, and Tits, whose habit it is to place seed in their feet and chip away at it with their bills.
Saying this, and because of the dietary requirements of softbills, any wild bird food provided should be low in iron. This is, crucially, because certain softbill birds can be affected by an iron storage disease, which affects the liver and prevents it from it’s a normal filtering function.