Elves possess predominantly avian features—feathers, leathery legs with knees that bend backwards, owl-like beaks, penetrating eyes. Specific feather styles and color patterns vary considerably between subspecies. Though elves do have hollow, lightweight bones and large feathers on their arms, they are incapable of natural flight.
Their preference is for a vegetarian diet—whole uncooked grains, tender greens, and berries—but they will eat insects if necessary. They can digest nearly any food available to humans and halflings, but culturally it is taboo to eat any other meat.
All elves feel a powerful preternatural urge to migrate between sites of great magical power, performing various tasks necessary to maintain them (resetting traps, cleaning up corpses, recharging magical devices, recasting spells, replacing artifacts, repairing architecture, etc.). Many believe these migrations are necessary for their longevity. Each elf has his or her own innate migratory pattern, criss-crossing a region and returning "home" each spring at the time of the Homecoming Festival.
Since they are constantly on the move, elves only see their friends—or spouses—once a year, when their paths cross. As such they have developed a rich oral tradition of romantic poetry, composing long ballads, at once extravagant and austere, about true love, true friendship, and fidelity. They also have a written tradition, but their writings tend to be more academic in nature, and authorship is shared among many elves who might visit the same temple or library at different times over the course of a year. Some famous elvish historians have collaborated for centuries without ever meeting face-to-face.
Elves do have permanent buildings and towns, but without permanent residents—the same houses, storerooms, and outposts will be used and maintained by various elves over the course of a year (each stopping for a few days at a time), who communicate with each other in writing. (Note: Because their knees bend backwards, they do not use chairs, and thus their structures are mostly devoid of chairs, benches, etc., except those kept in the interest of hospitality toward other races.)
Elves are unswervingly monogamous, not even remarrying after the death of a spouse. Marriages are arranged by committee, a process conducted over written correspondence that may take ten or twenty years. It is required that the would-be couple's migratory paths cross at least once a year.
At each full moon, a fertile female elf lays an egg about eight inches across and ten inches tall—but unless she has been with her husband within the last month, the egg is unfertilized and she will leave it hidden somewhere in the wilderness. (To elves' horror, halflings consider elf omelettes a delicacy.) Pairs of eggs are rare, and pairs of fertilized eggs (that is, twins) are considered highly portentous.
A fertilized egg will be carried by the mother in a sling until it hatches about four weeks later. Until the child is of age and sexually mature (that is, around fifty years old), it will travel with its mother, learning from her. Unless raising a child or traveling with other races, elves travel alone.
Perhaps due to their migratory lifestyles, elves tend to be the least greedy of the dominant races. Rather than hoard treasures, they desire to return them to their "rightful" homes. They consider themselves (self-importantly, according to most everyone else) the Stewards of the Realm. When the humorless elves talk about their "duty", others often have a hard time keeping a straight face. ("Ha! You said doody!")
Elves are mostly anarchistic, but when it is necessary to make community decisions, all elves of age can participate in direct democracy via written correspondence. A decision can often take years, and in practice its implementation depends heavily on the interpretation of the individual elves.
The unwavering patience and conservatism of elves is a frequent source of frustration to others—especially humans, who huff and puff about inevitable "progress".
Elves as Player Characters
It's quite rare to find an elf traveling with an adventuring party. The most likely explanation is that, for any number of reasons, a mixed party would accompany an elf on the migratory path. But there are some reasons an elf might not migrate at all: A wayward elf could find ways to dull the migratory instinct (e.g. with a steady intake of alcohol). Very rarely, elves are cursed with a natural lack of the migratory instinct, a condition treated by other elves with pity and sometimes disgust. (Without the instinct, an elf would likely feel lost and purposeless, rejected by nature or the gods.) Or, an elf might consciously (albeit with much difficulty and pain) neglect his migratory duty in service of a more urgent cause (say, saving the world from a dragon or Dark Lord).
What kind of elves inhabit your world?