While I am no scientist I believe that the closest relative to dragons are hummingbirds.  Granted there is a disparity in size but just because an chihuahua and a Great Dane are different in size does not change that they both are dogs. Hummingbirds and dragons  can both fly forwards, backwards, sideways, and even upside down and backwards in a pinch and most of all they can both hover.  This means that unlike other birds the hummingbird gets force from both the down-stroke and the up-stroke of their wings.  Some sources I have read say that there is an equal force from both strokes others say that the up-stroke isn't quite as strong.  Either way this is impressive and unique to humming birds, dragons and insects.  Prehistoric flying dinosaurs like the pterosaur or pterodactyl  are believed to only glide  or only have short powered flights modern day reptiles can only glide by spreading out skin to act like a kite or parachute.  Thus I feel that dragons and hummingbirds are more closely related than dragons and any reptile and hummingbirds and any other birds but swifts.  Next I am going to continue my investigations  by checking if feathers and dragon scales are perhaps modifications of the same materials.
P.S. If you are into symbolism as am I the motion of the hummingbird wing is a figure 8 or the infinity symbol.  Incidentally dragons are long lived and often represent long life and eternal power in art.