Bill Hilton's commentary on aging is very useful, but he neglected to
address the question "how about after the second year?" To my knowledge,
there's no way to age birds other than plumage (in eagles, gulls, species
that take numerous years to mature), once they are past their second year
(or past their first year in most birds). All those lab techniques are to
distinguish first-year from after-first-year birds. In some waterbirds,
bursa of Fabricius size is correlated with age after the first year, but it
gets pretty sloppy.

Basically, skull pneumatization in passerines and bursa size in
nonpasserines (we also look at this in passerines, as immature passerines
with completely pneumatized skulls in winter may still show bursa
development) is how we age birds we prepare at the Slater Museum.

Bill didn't mention the very important characteristics of feather shape and
feather wear. Many young birds have differently shaped tail feathers, and
sometimes wing and contour feathers, from older birds (see Pyle et al.
1987, Identification Guide to North American Passerines, Slate Creek Press,
Bolinas, CA, for passerines). Typically juvenile birds grow their first
wing and tail feathers in early or midsummer and don't molt them until
their second summer or fall (plenty of exceptions, however, and works best
for nonpasserines). Adults molt later in fall (and may have stronger
feathers), so the young birds' wing and tail feathers are always more worn
than those of the adults, until the second-year molt. This allows easy
distinction of first-year individuals in many kinds of birds, for example
crows (and you thought they all looked the same!).

There are lots of other idiosyncratic methods, e.g. the corrugations on the
bill of young but not adult hummingbirds.

Dennis Paulson