Varient Avariels + Modified Winged Template

Varient Avariels: Much in the same way that dwarven thunder twins, a four-armed sahugain or a blue goblin is periodically born, there is a small chance that an elf will develop avian traits during puberty, becoming an avariel at adulthood. Elves that become avariels acquire the winged template. About one in a hundred elves, never including sea elves or drow, develop these traits by the time they reach maturity.

MODIFIED WINGED TEMPLATE
"Winged" template is an inheritated template that can be added to any corporeal creature (except elementals, oozes and plants) that doesn't already have a fly speed. A winged creature has all the base creature's characteristics except as noted here.
Size and Type: Animals become magical beasts.
Speed: A winged creature gains a fly speed equal to twice its base land speed with Good manuverability.
Special Qualities: A winged creature gains the special quality described below.
Improved low-light vision. If the base creature already has a form of low-light vision, that ability improves by two steps instead (to a maximum of Superior low-light vision).
Abilities: +4 Dex, +2 Wis
Skills: A winged creature gains a +4 racial bonus on Balance, Jump and Spot checks.
Feats: A winged creature gains the benefits of the Altitude Adaptation and Cold Endurance (Frostburn, pg. 45) feats.
Challenge Rating: Base creature's CR +1
Level Adjustment: +2

Sight distances among those creatures with low-light vision:

• Low-light vision (x2 distance)
• Improved low-light vision (x3)
• Greater low-light vision (x4)
• Superior low-light vision (x5)

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I wanted the winged template, which originally appeared in the Savage Species manual, to outfly other templates with an equivalent LA that grant flight, but don't focus primarily on it.  The benefit of the feats help with high altitude flight.

I'd put the odds of this kind of avariel being born at 1% to make for a clean 1d100 roll in the case a DM makes a player roll for it.  Also, having avariels emerge in this manner has a few benefits:

    1) gives one more choices for your avariel
    2) removes it from the "menu" of the Alter Self spell
    3) Avariels won't die out as long as surface dwelling elves remain
    4) these Avariels will live longer due to having a different age chart

It could be a blessing bestowed upon them by Aerdrie Faenya... or an effect of race-altering high magic along the same vein that first altered the dark elves into eventually becoming the drow (FR)... though done with different intent.

Also: The odds of a four armed sahugain being produced is 1 in 200.