"If you were to take a group of people and shrink them down to the size of an English Sparrow, they would loose much of the individual characters that you use for recognition of individuals. The distinctive characters would still be there, but much harder to see, since they would be smaller.
"Birds recognize each other by their voices or calls. They can identify mates, parents or offspring by voice, much as a blind person might do. During courtship and pair formation, birds learn to recognize their mate by 'voice' characteristics, and not by visual appearance."
Pamela J. Pietz of the Northern Prairie Science Center in Jamestown, North Dakota, considered two parts of the question--how do we tell animals apart, and how do they distinguish each other. Her reply follows:
"Humans can tell individual animals apart in many species, but it takes some familiarity with the species of interest. For example, researchers can recognize individual humpback whales because each whale has a unique black-and-white pattern on the underside of its tail flukes. Biologists studying great right whales (such as Roger Payne in Argentine Patagonia) recognize individuals by the unique patterns of whitish growths, called callosities, on the whales' heads.
"Craig Packer and Anne Pusey of the University of Minnesota use whisker patterns to tell individual lions apart in their African research. For short-term identification, some researchers use scarred skin, missing feathers or other physical anomalies to tell animals apart. Researchers record tears and notches that they observe on the edges of elephants' ears to distinguish African elephants from one another, for instance.
"The common aspect of all these recognition systems is that the people who devised them spent lots of time looking closely at lots of individuals of one species. It is likely that we could recognize individuals of many species if we spent enough time observing them carefully. Of course, individuals of some species do look more alike than individuals of some other species. For example, some invertebrates have reproductive systems that lead to many individuals being very closely related to each other genetically (much like identical twins in humans). The less genetic variability there is among individuals of a species, the more closely they will resemble one another.
"We should also remember that visual markers are not the only possible way to tell individuals apart. Humans tend to rely on visual cues more than other types of cues because our vision is more highly developed than our other senses.
"Individual animals of a given species probably can tell one another apart as easily as we can tell humans apart, but they may use sound, smell, and other senses instead of, or in addition to, vision. Birds are strongly visually oriented (that's why they are so colorful), so they may use visual cues to recognize individuals; they also have excellent hearing, however, so they may respond to differences in individual voices as well, much as humans do. Reptiles use chemical signs (akin to 'smell') to gather information about their environment, so they probably likewise rely on chemical signs to tell individuals apart. Some animals see beyond the visible light spectrum (bees and some birds see ultraviolet wave lengths), and some animals hear sounds that are too low (e.g., elephants) or too high (e.g., dogs) for humans to hear. Thus, some animals may use cues to tell each other apart that are not available to us.