Several hypothese have been advanced
concerning the functions of avian
head-scratching including preening of the head
plumages, preading of uropygial oil on head
feathers and relief of local irritations on the
head (Burt & Hailman, Ibis 120:153-170,
1978).
A fourth hypothesis holds that
head-scratching may help to alleviate pressure
differences in the middle ear or eustachian
tubes. This idea derives from Andrew's (Br. J.
Anim. Behav. 4:85-91, 1956) observation that
head-scratching often is directed at the
external opening of the ear, and that it
sometime is associated with yawning. However,
since Andrew's study I have found no published
data that confirm this relationship between
head-scratching and yawning.
On 20 Mar. 1978, I watched 15 to 20 Black
Skimmers( Rynchops niger) head-scratching (by
bringing one foot directly up to the head, under
the wing) and yawning (by opening the bill for
1-2 sec) as they stood on a beach by Lake
Okeechobee Florida.
If one of the functions of head-scratching
is to help clear the eustachian tubes ,as y
awning presumably does, then one would expect
yawning to be associated only with
head-scratches that contact the head near the
external ear opening; scratches that contact the
head in other areas, such as near the bill or on
the top or back of the head, would presumably be
in response to other stimuli and should not be
associated with yawning.
I observed 26 different bouts of
head-scratching among these Skimmers:1 0 of 15
scratches (67%) that contacted the head near the
ear were followed within a few seconds by
yawning,but none of 11 scratches that contacted
the head elsewhere were followed by
yawning.
This relationship of ear directed scratches
with yawning is significant ( x2 = 11.92, df =
1, P< 0.001), and although it does not prove
the internal pressure hypothesis it is consisten
with the hypothesis and Andrew's original
observations and suggests that hidden internal
changes may affect the occurrence of
head-scratching.
Le comportement des oiseaux de se gratter la
tête avec l'aile modifie les pressions
dans l'oreille moyenne, en ouvrant l'orifice
externe de l'oreille. Cela est souvent
associé à des bâillements,
qui sont connus pour ouvrir la trompe
d'Eustache.
En mars 1978, SR. Robinson observa un groupe
d'une quizaine d'oiseaux (Le
bec en ciseau noir) Rynchops niger au bord
d'un lac de Floride. Il nota 26 fois ce
comportement de frottement de la tête avec
l'aile. Dans 67% des cas ce mouvement
était suivi d'un bâillement. Une
coincidence parait bien peu probable.
