How do falconers commonly measure the primary feathers of birds?

Falconers commonly measure the primary feathers of birds using the ornithological numbering system. This system assigns a specific number to each primary feather starting from the outermost feather and moving inward. This allows falconers to consistently and accurately identify and refer to the feathers when discussing their birds, performing calculations, or providing care.

The other options are not how primary feather measurement is typically approached. Counting from the outermost feathers inward can be part of the process, but it does not provide a standardized way to refer to each feather. Measuring the length of the feathers is a method that may be used in some contexts but does not relate to the standard nomenclature for identification purposes. Lastly, evaluating feathers based on their color intensity is not a relevant metric for measuring or identifying primary feathers in falconry.