It is common knowledge that the male downy woodpecker has a red patch on the back of its head. Female downies lack such a patch. Knowing this, you can easily tell a male from a female downy woodpecker.
However, did you know that you could distinguish between individual downy woodpeckers? Remarkably, this is possible. It seems that the black and white pattern displayed on the back of the head of each downy woodpecker is slightly different from that displayed by any other downy.
With that in mind, if you photograph or sketch the patterns displayed on the napes of each downy woodpeckers seen at your feeders, you can learn all sorts of neat things regarding the downy woodpeckers that feed in your yard. You can determine how many individuals use your feeders. It would also be possible to discover if some birds are more dominant. You name it.
This technique is similar to the one used by biologists to differentiate between individual zebras. In the case of zebras, each animal has its own distinctive pattern of black and white stripes.