 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 is 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 . 
