IT'S WIDELY BELIEVED THAT anyone who puts up a bird feeder, especially in winter, is duty-bound to keep the feeder full day in and day out. This notion is based on the belief, also widely held, that birds become "dependent" on feeders, thus will starve if we humans don't keep the feeders full.
As is so often the case, the reality is more complicated than that. Research into the habits of birds that visit bird feeders indicates that they use the seed we supply primarily as a supplement to other food sources. They probably rely more heavily on this supplemental food in winter and spring, because calorie needs are high in those seasons. (In winter, they need the extra calories to stay warm; in spring, because they are nesting and raising babies.) However, birds forage widely for food, so if the feeder in one garden happens to be empty they usually know of other places where they can get something to eat.
Think about it: Nature never provides a constant supply of food from any one source. Seeds of one patch of plants mature and are available for eating for a week or so, get eaten, and disappear. The birds move on to another food source. (This is one of many reasons why it's important to have diversity in plantings.) The bird that became dependent on one particular type of seed from one particular patch of plants wouldn't survive its first week!
Birds as a group do not become dependent on feeders in the sense that they cannot survive without them. Mom Nature is not that dumb.
But does that mean you can go to Florida for a week in January and not worry about keeping the feeder full? Well, it depends.
How birds use feeders depends on the species. Banding studies show that members of the finch family tend to move around a lot; the finch you see at your feeder today is probably not the same finch you saw yesterday, even if they do look alike. These birds deal with food shortages simply by moving on to the next food source, be it a patch of plants or somebody else's feeder. In contrast, chickadees tend to be faithful to a particular territory, especially in winter. Throughout the winter the same small flock (usually about six birds) will visit your feeder all day every day as long as you keep it full. But even chickadees are prepared to deal with shortages; a lot of the seeds they take from your feeders are not eaten that day--instead, the seeds are hidden for future use. An individual bird can remember thousands--yes, thousands--of spots where it has hidden seeds away.
This means that the majority of birds will survive even if you leave the feeders empty for a week during a cold snap. There's no evidence that a population of birds has ever been wiped out because somebody forgot to fill her bird feeders.
Individual birds are another matter, however. The presence of a well-stocked bird feeder can make the difference between life and death for a bird that would have died without human intervention.
Sick or injured birds are an example. These birds will often hang around feeders because they are not able to feed themselves as easily as other birds are. Sick birds usually die regardless of whether they have access to food, and their presence at feeders may be a threat to healthy birds. Injured birds may heal, however; many people have heart-warming stories of one-legged or otherwise disabled birds that have managed to live, sometimes for years, through the kindness of humans. These are birds that Mom Nature, who is smart but not always kind, would have thrown out of the gene pool. They survive only because some humans have an endearing soft spot for weak and vulnerable creatures.
Another example is what happens during a patch of especially harsh weather. Mom Nature would let some of the weaker birds freeze or starve. Not so we soft-hearted humans; when we make food readily available, even in the worst weather, we help the marginal creatures get by.
So ... do you need to keep those feeders full or not? The less-than-simple answer is that it depends on how hard hearted you are. Some people would, and do, argue that providing supplemental food interferes with Nature's plan, helping weaker animals to survive and breed when it would be better to let them die. This is harsh, but it may be true.
Efforts to keep individual birds alive are often destined for failure. Nature has plenty of ways to prevent a species from overpopulating. If starvation doesn't bring the population down, disease (a very real danger among feeder birds, who pass diseases around while sharing space at the feeder) may do the trick. And if not disease, hungry predators (like that hawk who will probably be attracted to your feeder at one time or another) will take care of the problem.
On the other hand, some of us get emotionally attached to "our" birds. And we think it's worth making the effort to feed them, even if it only gives a few individuals a few more days or weeks (but possibly months or even years) of life. Humans are just funny that way.
If you're in this camp, you'll want to try to keep your feeders full. However, if you do happen to miss a day or even a few days, you can rest easy in the knowledge that the vast majority of your feathered friends will make it through just fine.