The normal foods that wild birds eat - insects, nuts and berries - are gone by the middle of winter. So what do they survive on until spring? Do they store food? I’m not counting bird feeders, which would not have existed in pre-human times. And what happens when there’s a foot of snow on the ground? What do they eat then??
There’s still berries in the winter. A lot of shrubs have persistant fruits, meaning that they last until either a new flower bud knocks them off or something eats them. I’ve seen a lot of viburnum, for example, still holding their shrivled fruits in March or April. Likewise with crabapples. Also, wild grasses and other plants will continue to hold their seed heads again until something takes the time to eat them or the seeds are dispersed. Certain other birds will eat the seeds from pinecones. Insect eating birds head on south for the winter, as it is rather hard to forage bugs under a foot of snow. However, seed and fruit eaters may hang around depending on what kinds of seeds and fruit they eat, although they do typically plump up as much as possible to get them through the lean winter and to help keep themselves warm.
Winter’s not easy on the little feathery guys, but they do manage.
Actually, some birds, like nuthatches and chickadees, can find insect larvae and pupae in bark and buds throughout the winter. And woodpeckers of course eat insects sheltered inside the tree trunls and branches.
There’s more bird food around in winter than there appears to us. The birds just know how to find it.