This is interesting stuff. I have seen crossbill skulls. It is not only the
horny bill covering which is crossed but also the bones within them.

Bones can change position based on the stresses placed upon them. Is the
resulting crossed bill the result of genetically programmed behavior -
constant intermittent stress placed upon the bill by feeding and bill
manipulations? Or are the bones genetically programmed to cross and the
young bird just helps the process along?

Dave Parent, Freeland WA dpdvm at whidbey.com

-----Original Message-----
From: TWEETERS-owner at u.washington.edu
[mailto:TWEETERS-owner at u.washington.edu] On Behalf Of David Beaudette
Sent: Thursday, January 01, 2004 1:39 PM
To: Tweeters Messages
Subject: Crossbill bills

Greetings,

This came through on Montana Birding today. Interesting...Any comments?

Dave Beaudette
Wenatchee,WA
drtbrdr at earthlink.net

Subject: How Crossbills get their crossed bills.
From: Judy Hoy <bwrehab AT mtwi.net>
Date: Thu, 1 Jan 2004 00:14:35 -0700

Happy New Year mobsters,

Here is an observation by Western Montana rehabers that may interest
those of you who did not already know this.


HOW CROSSBILLS MAKE THEIR
BILLS CROSS

I have raised several hatchling Red Crossbills and so have had
the opportunity to observe them closely as they matured. As hatchlings,
their bills are wide, similar to other hatchlings of the Fringillidae
Family. As fledglings, their bills are quite cone shaped somewhat like
a small Grosbeak bill, with the top and bottom bills coming to a point.
The bills are nearly even in length, with the upper bill being very
slightly longer than the lower. A fledgling Crossbill retains this
shape and length through the period when they are following, and are
still being fed by the parents, after fledging from the nest. During
this time period of several days, the fledglings learn by observing the
parent birds and each other, how to find and procure food for
themselves.
The fledgling birds then reach a developmental stage, at which they
are attempting to extract pine seeds from conifer cones for themselves.
At this time, the tips of both the top and bottom bills begin to grow
dramatically. This appears to happen as a result of stimulation of
certain hormones, which trigger a sudden growth spurt at the anterior
of the ramphotheca.
Birds in captivity have been observed in this developmental stage by
wildlife rehabilitators to exhibit the following behavior. The birds
select a protruding knot or the butt of a small broken branch on the
long branches which serve as perches in the flight room in which the
birds are kept prior to release. While the tips of the bills are
growing, the young Crossbills twist the top bill to one side and the
bottom bill to the other, using the selected protuberance on the branch
to pry against. As the tips of the bills lengthen, the birds pry and
twist several times per minute, except when they are eating or
sleeping. Once the bird begins to pry the top bill sideways, it always
twists the bills the same way each time. After two or three days, the
bills are fully grown and remain crossed. The fledglings can then use
their newly reformed bills to force open the conifer cone scales and
hold them apart while they extract the seeds with their dexterous
tongues. The young crossbills usually twist the mandibles in such a way
as to cause the top bill to go to the right and the bottom bill to go
to the left. A smaller percent twist the top bill to the left.
This fascinating phenomena in the life of a fledgling Red or
White-winged Crossbill does not appear to be well documented. I could
not find this mentioned in bird behavior discussions in books or on in
literature searches on the web. If anyone has, I would appreciate
knowing the source.

Judy Hoy_______________________________________________
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