Cocker Spaniel. This is the answer.
My Springer is only 35lbs, but we train with folks with field-bred cockers and those dogs are lightning bolts! They will tear up any upland field you throw them at. They'll retrieve geese.
(The good ones are also absolute psychos, but hey, worth it.)
My friend has an English Field Cocker, outstanding dog! His is a female that I doubt is much over 20 pounds, but looks like males might go over your minimum weight for flying. She's fearless, goes ape s&#t around birds. Worth a look at least.