Discussion
Shrikes have often been compared to birds of prey (particularly falcons)
because of their morphological and behavioural similarities. The shape
of the shrike’s beak bears many striking resemblances to a falcon’s
beak, complete with hook and tomial tooth. The slightly protruding
position of the eyes facilitates binocular vision and the gives an
appearance of a heavy head (Cade 1967, 1995, Schön 1996).
Shrikes kill prey with their beaks and carry the largest prey with them.
They then hold them with their feet on a piece of vegetal support to
manipulate them or impale them on a thorn bush to dismember them with
their beak. In Southern France, the northern extent of its range, the
diet of the southern grey shrike consists mainly of insects. Museum
specimens possess a powerful beak with an average length of 15.96±0.21
mm which is slightly longer than on specimens found in Spain (13.96
±0.64 mm, Gutiérrez-Corchero et al. 2007).
It is well known that bill size and shape in shrike vary as a function
of geography and climatic factors and may result in varying degrees of
suitability for certain species of vertebrate and arthropod prey (Strong
1901, Sustaita & Rubega 2014). Shrikes with narrower culmen and longer
hook tips produce lower bite forces than those with thicker culms and
shorter hook tips. It could be hypothesized that the Iberian grey shrike
has adapted to feeding on arthropods on the Mediterranean plains.
Only few bristle measurements have been carried out on the genusLaniidae . In long-tailed shrike, Lanius schach , and great
grey shrike, Lanius excubitor , black barbed bristles can be up to
11 mm long in the schach, and 7 mm in the excubitor. They also possess
numerous shorter, barbed, black nasal bristles that curve over the sides
of the beak and nostrils, before merging into loral half-bristles
(Stettenheim 1973).
In the Iberian grey shrike, the rictal bristles are clustered into
bundles of four or five strands with a maximum length of 10.27±0.12 mm.
They can block wings or serrated leg parts when the bird is manipulating
large insects of the genera Orthopterae and Mantidae (Guillaumot 2021)
and are likely to play a role in protecting the birds’ eyes when
transporting large prey. Another possible function of rictal vibrissae
that remains utterly unexplored is the detection of movement of beaked
prey, providing a sensory function similar to whiskers in some mammals
(Cunningham et al. 2011).
On the loral zone there are several types of bristles, ranging from the
basic structural plan of the feathers, from which they are derived, over
variously branched semi plumes to stiff, unbranched bristles. The
bristle spindle is pointed and dark coloured especially at the base.
This dark coloration is caused by a large deposition of melanin which
increases the strength and abrasion resistance of feather keratin
(Bonser 1996) and also contributes to bristle rigidity (Stettenheim
1972, 1973). Bristles around the nostrils and ear openings can protect
these openings (Stettenheim 1973).
Above the eyes, shrikes have a row of small tight feathers that form a
white eyebrow. While the skull of shrikes is quite similar to that of
typical passerines, except that the distance between the eyes is
slightly increased, the position of the eyes in grey shrikes is somewhat
peculiar in that their relatively large eyes protrude slightly on the
side of the head while the adjacent parts of the head are slightly
depressed (Schön 1996). This thick frown forms part of the range of eye
protection feathers.
Shrikes are capable of rapid axial head rolls which rotate their prey’s
bodies around their own necks creating accelerations equivalent to about
6 g. These accelerations are sufficient to kill mammalian vertebrate
prey and by causing pathological damage to the cervical vertebrae and
spinal cord (Sustaita et al. 2018). Similarly, the Iberian grey shrike
uses these rapid head turns to dislodge broken or irritating insect prey
particles after beak-handling.
Analysis of the rictal plumage of the Iberian grey shrike in southern
France shows that the species has adapted to the native trophic
conditions. Complementary morphological studies and mitochondrial DNA
analysis of the entire population, including the Iberian Peninsula and
France, could refine the knowledge of the species in the north of its
range.