@winterflower18:
Different birds eat different things. Just as with all the other really large animal families.
Geese are apparently mostly herbivores, though the notes on Brant Geese (which is what our big friend is) on Wikipedia are kind of fascinating: Apparently they traditionally mostly eat seagrass (at least on the nothern end of their migration?) but some groups have been adapting to changing ecosystems by, for instance, moving inland and eating stuff other geese eat.
I gather that the diet of sparrows in China in particular, on the other hand, is a matter of some recent-ish human historical importance, because early communist China tried to go on an extensive campaign to drive them to extinction, as they were considered "pests" that ate rice. This, like many such heavy-handed ecological interventions, went very poorly XD because they also eat a good number of rice-pest insects, notably including locusts—and so yeah, plagues of locusts, fun times. o_o;
Which is to say sparrows, at least, are indeed the sort of grain/insect omnivore birds you're thinking of.
But there's also obviously carnivores; the various birds of prey: Hawks, owls, vultures, kingfishers, penguins... (There's probably something that
only eats insects, but I don't know what it is.)
(You would expect sparrows to be the better bacon-eaters of the two, then, incidentally, but who knows; I certainly don't. If they're kept as pets in China as much as is implied in this comic, presumably someone knows though.)