Can some of you clear up a question? Are there accurate lab methods for
determining age in dead birds besides plumage characteristics? Mammals
have cementum annuli, fish have osteoliths, trees and bivalves have growth
rings. There must be a similar method for birds? This question arises
out of a weekend discussion with a friend about bird necropsies, and
neither of us could think of a way. Can anyone relieve us of our
ignorance in this matter?

Mike Smith
University of Washington
Seattle, WA
whimbrel at u.washington.edu

-------------------------

Date: Wed, 1 Jun 1994 16:48:35 -0500
From: "Bill Hilton Jr." <HILTONB at VUCTRVAX.bitnet>

Although, as Mike implies, plumage ( especially color and degree
of wear) is the most commonly used ageing character, bird banders
use a few other non-lab techniques that may or may not be helpful
for any given bird species.

SKULLING
The most common of these is "skulling," i.e., examining the
bird's skull for degree of pneumatization. A bird's skull is
"two-layered," with the outer and inner surfaces being separated
in adults by air space; the two surfaces are kept apart in adults
by vertical "columns" of bone, while in the juvenile there is
instead a layer of soft tissue. When crown feathers are moistened
& pulled back, the skin is thin enough to allow a view of the skull
bone. In the adult, the vertical columns look rather like tiny
polka-dots; this stippling is absent in recent hatchlings, but as
the bird ages, columns begin to appear along the central
cranial fissure, the back of the skull, and just behind the base
of the beak. With magnifying glass, skill, and luck, it is often
possible in autumn migration (or even in Year Two) to determine
by skulling whether a bird is young or an "adult." Woodpeckers and
birds with crests are notoriously difficult to age by skulling, and
some species pnematicize so quickly that the technique is suspect.
With a dead bird, it is easy to slit the skin on the skull for a really
good look at the degree of pneumatization.

EYE COLOR
For some species the eye color is a good ageing character.
Most Rufous-sided Towhees have brown irises as juveniles and
red eyes as adults. (Florida birds, however, have yellow or white
irises as adults.) Under a hand lens, the iris in many species
appears "muddy" in juveniles and "clear" in adults, but I'm not
sure this would work very well with a dead bird.

GAPE & BILL LINING
The tissue in the corner of the "mouth" in young birds of many
species is soft and yellow, becoming harder and gray in adults.
The rate of color change varies with the species, and never
occurs in some. In autumn, the best way to age Ruby-throated
Hummingbird females is to carefully pry open the beak a little;
older females have a bill lining that is whitish or grayish or
bone-colored, while young of the year show at least some yellow.

BURSA
Lastly, in gallinaceous birds and waterfowl, the Bursa of
Fabricius can be observed just inside the cloacal opening of
young birds; this gland (which plays a role in building immunity)
is resorbed in adults. (This ageing method explains why duck
biologists never wear dress clothes while banding!)

These are four general ageing techniques that spring to mind.
Anyone have others that don't require dissection, electron
microscopes, or blood titres?


HAppy Birding!

BILL HILTON JR.
"The Piedmont Naturalist"
Hilton Pond
1432 DeVinney Road
York, South Carolina 29745
(803)684-0255
hiltonb at ctrvax.vanderbilt.edu