Why do sparrows leave for a month and then come back?

Answer 1

There are two possibilities for why small birds leave (or appear to) for a few weeks.

One is that there is often more food at certain times of the year in surrounding farmland than there is in your garden.

The second is that all birds have to replace their feathers over a period of time. They are vital for flight, warmth and often for display, but do become worn. In raptors and other birds that soar, this can take 3 or 4 years as one feather is replaced at a time. In other species birds will undergo a complete moult over a period of a few weeks. Ducks and geese are an example. During this period they can't fly, and will hide from predators, and will not be obvious to even the keenest birdwatcher. Late summer and early autumn can be a lean time for birders!