This post continues the discussion of the Seven methods of identifying birds.
[Answer to Quiz 2: American Avocet]
In several families of birds, identification is aided or clinched by noting either individual feathers or feather groups. Such is the case for gulls and shorebirds. Sometimes in-field study of the specific feather group patterns can determine age, which then eliminates other certain species at the same or different age.
When you are discussing scapulars or tertials or molt, you are using this method of bird identification.
Quiz) The internal pattern on the tertials of juveniles of this long-billed shorebird easily separates these two species.