Just like virtually all other creatures on earth, birds can learn from from experiences and react accordingly. Remember learning about Pavlov's dogs in school? Just in case you don't, here's a little background. Ivan Pavlov trained his dogs to salivate at the ring of a bell by simply ringing a bell each time before he fed them. In time, the dogs learned that the bell meant that it was time to be fed, and would begin to salivate.
Just as dogs, and a good number of other animals (most, if not all animals), including the ordinary wild bird, can learn from repeating circumstances. I will share two examples. The first is a well-known fact to bluebird junkies such as myself. Bluebirds can be trained to come to the sound of a whistle. This is relatively easy to accomplish. All you need do is locate the favorite perch of a bluebird (which is relatively easy if they are nesting nearby), and place a dish of mealworms nearby. After the bluebirds find the worms (which will happen in a very short ammount of time), every time you fill the dish just blow a whistle, ring a bell, or make some other sort of distinct noise that can be heard a good way off. In no time flat the birds will learn that the whistle means chow time, and will begin showing up at the feeding site as soon as they hear you blow it. In some cases, the bluebirds will come very close, or let you get very close to them.
My second and last example comes from personal experience. In fact, I had unwittingly 'taught' the birds when to expect me to fill up the bird feeders. As most of you probably don't know, I only feed the birds (with the exception of hummingbirds) during the winter. Even in the winter when I do feed, I put out less seed on days when there isn't snow on the ground. However, on days when it would begin to snow heavily in the afternoon or evening, I would give the feeders a refill. Little did I know, the birds eventually caught on to this pattern. Now that March is over and April is rapidly approaching, I hadn't fed the birds in upwards of two weeks. But on one particulary cold day last week, the snow began to fall around 4 pm. It was falling exceptionally heavily, and the forecast threatened a half inch of snow. As was passing through the kitchen, I glanced out the window, and the sight that I beheld suprised me more than a little. It had hardly been snowing for a minute and a half, but already the birds- many of which I hadn't seen since I stopped putting out the seed a few weeks earlier- were already gathering, looking for food. A pair of northern cardinals hooped about below the feeders, as they usually did, in search of fallen seed. A song sparrow joined them. It was then that I decided that it was high time to use up my very last soop of birdseed. I honestly don't know why I had saved it, but now I am glad that I had. I promptly put out the last little bit of seed, and before the day was over a downy woodpecker, red-bellied woodpecker and a pair of Carolina chickadees had showed up for the feed.
Some habits are hard to break, even in the bird world. If it snows, show up for seed. Even weeks later, they still somehow remember when the free food will show up.