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.