A colleague of mine asked the other day what the upturned wing tips on many modern commercial aircraft are doing, and if they have any similarity to the split wingtips used by many birds. A full explanation would take pages, but here's a quick and dirty one:
There is a price on producing lift called 'induced drag'. Induced drag can be modeled a few different ways, but basically it involves some efficiency-reducing downwash at the wingtips involved in the formation of the wingtip vortices. A wing without a tip does not suffer from induced drag, as a result, but in most real situations wings obviously must have tips (in a wind tunnel you can build a wing that goes from wall to wall to eliminate induced drag effects).
An upturn of the wingtip can, in certain speed regimes, reduce induced drag. Splitting the wingtip into smaller, high aspect ratio wings can also reduce induced drag. Fixed wing aircraft can be designed with the first of these two tricks. Many birds use both: they spread the primary feathers in both the transverse and vertical planes - this means they get the multiple-tip bonus and the vertical displacement effect.
Induced drag is particularly problematic at low speeds, so these tricks are best for slow soaring, landing, and takeoff. Of course, in a fixed-wing aircraft, the winglets are set in place and cannot be differentially deployed. In animals, however, slotted wingtips are only deployed when it is most useful - that is, at low speeds.