It has been an easy winter in NJ, so they've had access to natural food sources pretty much all winter. Many eat seeds from plants and flowers that bloomed the summer before.

So I think they're OK.

I'm not sure if there is a designated time to stop feeding. Many people feed year-round; some stop after the thaw.

Birds do not depend on human feeding. They will find food in the berries and seeds of natural plants. It is nice to put food out after a heavy snow, but my understanding is that human feeding is not necessary. It is also blamed for the spread of some disease like finch-eye conjunctivities and salmonella. Natural foods spread birds around which helps to minimize disease. Plants also provide cover for birds and nesting sites. Cover gives birds a safe place to hide from predators. Often predatorial birds will notice a concentration of songbirds and will post themselves near feeders. My understanding is that the more important thing to provide is water.

As far as feeding seeds like sunflower and thistle, you can feed year-round if you like. One advantage of summer feeding is you might draw in interesting birds like rose breasted grosbeaks, or indigo buntings, which you wouldn't see feeding in winter.

Some say feeding birds in late winter/early spring puts them in better shape for the breeding season. Here in Ohio, its still cold (and we're expecting 3 to 5 inches of snow!) so i'm keeping the feeders filled for awhile.

In Winter, birds get only a minimal percentage of their food from feeders, though it may seem otherwise. They're incredibly adept at finding what they need. God just kinda made them that way. But, my advice, stop when you wanna stop. If you're feeding them to feed them and not also to enjoy watching them, it shouldn't really matter much to the birds. I'd say, feed 'em until things really start to green up and grow. Just my thought though.

I signed up to receive email's from the National Wildlife Federation, and so saw a link related to topic. It says that feeding in March can make things easier on the birds since most natural sources are at their lowest levels at this time.

I still have a bird feeder, but am starting to believe supplying food naturally through plants is the best method to avoid disease spreading from overcrowding. The link did say the birds aren't dependent on the feeder food.

Here is a link that might be useful: NWF Wildlife link about March tough month for birds