Flight Feather - Wing Formula

A wing formula describes the shape of distal end of a bird's wing in a mathematical way. It can be used to help distinguish between species with similar plumages, and thus is particularly useful for those who ring (band) birds.

To determine a bird's wing formula, the distance between the tip of the most distal primary and the tip of its greater covert (the longest of the feathers that cover and protect the shaft of that primary) is measured in millimeters. In some cases, this results in a positive number (e.g., the primary extends beyond its greater covert), while in other cases it's a negative number (e.g. the primary is completely covered by the greater covert, as happens in some passerine species). Next, the longest primary feather is identified, and the differences between the length of that primary and that of all remaining primaries and of the longest secondary are also measured, again in millimeters. If any primary shows a notch or emargination, this is noted, and the distance between the feather's tip and any notch is measured, as is the depth of the notch. All distance measurements are made with the bird's wing closed, so as to maintain the relative positions of the feathers.

While there can be considerable variation across members of a species—and while the results are obviously impacted by the effects of moult and feather regeneration—even very closely related species show clear differences in their wing formulas.