In flight, the shape of the primary tips is often a function of several variables - mainly behavior.
Take for instance these two hatch year Herring Gulls, and consider the shape of their outermost primaries.
Notice the pointed, saw-toothed, shape of the primary tips on the first bird. This is quite typical in this age group. The second bird shows blunt-shaped and rounded primary tips. How could this be? Are we to throw away the usefulness of this criterion to help age 1st cycle gulls? Not at all. Just as with most field marks, we will always find exceptions. Another reminder to always use a suite of characters and never put too much stock in one single feature. The overall pristine wing coverts and immaculate flight feathers all favor a 1st cycle.
Okay, your turn. How would you age this Herring Gull?
The bicolored bill and pale eye are dead giveaways in this case. The rounded primary tips are but support for what is obviously a 2nd cycle type individual. This bird shows a rather retarded pattern, overall, with wing coverts and an uppertail pattern that look much like a 1st cycle.
A takeaway from this last photo should be, how would one age this Herring if it had a dark eye, or an all dark bill? What plumage features might we use? I'll follow up with a post on this very same aging question soon. Thanks for making it this far, and Happy Fall!