Introduction
Various studies have examined the variation in the size and shape of
rictal hairs in birds and proposed several hypotheses as to their
function (Lederer 1972, Stettenheim 1973, Cunningham et al. 2011).
Particularly in birds who use their beaks to immobilise their prey, such
as diurnal insectivorous birds of the “sit-and-wait” type or species
that capture their prey in flight, it appears plausible that rictal
hairs serve the function of eye protection or prey retention (Dyer 1976,
Cunningham et al. 2011).
Bristles provide eye and face protection from spiny appendages and other
prey threats (Dyer 1976, Conover & Miller 1980, Sherry & McDade 1982).
Facial bristles are modified hair-like feathers and are typically rigid,
stiff, and tapering (Lederer 1972).
Avian bristle feathers are found especially at the base of the bill and
nostrils, as well as in the lore (between the bill and eye), malar
(cheek, below the eye), and rictal (corner of the mouth) regions and
forehead, and can sometime take the form of “eyelashes” (Chandler
1914, Stettenheim 1973).
There is a range of bristle types, from the basic structural plan of the
feathers from which they are derived, through variously branched
semi-bristles, to the stiff, unbranched bristles seen around the slit in
many aerial insectivores (Chandler 1914, Stettenheim 1972, 1973 ). The
bristle rachis is generally pointed and dark in colour, particularly at
the tip (Stettenheim 1973). This dark coloration is caused by heavy
deposits of melanin, which increases the strength and abrasion
resistance of feather keratin (Bonser 1996) and also contributes to
bristle stiffness (Stettenheim 1972, 1973).
The Laniidae sp . are a very distinctive group of small to medium
size predatory passerines (14–27 cm), capable of preying on large
insects and small vertebrates (lizards, rodents, and occasionally other
birds). They are known for their distinctive behaviour of impaling prey
(up to 10 mm) on thorns and twigs for food reserves.
Their hunting technique is mostly sit-and-wait or perch-and-pounce.
Usually, they approach their prey by flying at altitudes of two to three
meters, sometimes briefly hovering over the prey, before quickly
descending to it.
The shrike’s beak is highly specialized for predaceous feeding. The
maxilla has a strongly decurved, sharp hook at the end with a
subterminal, tomial tooth on each side, and corresponding mandibular
incurvations, which work in opposition to the teeth (Cade 1967, 1995).
The shrike’s large, heavy-headed appearance is related to the cranial
adaptation of large jaw muscles required for a powerful bite (Cade
1995). Shrikes kill prey with their beaks, and they execute vertebrate
prey by biting the neck and disarticulating cervical vertebrates
(Sustaita & Rubega 2014).
Shrike eye position is elevated and probably contributes to a wider
binocular field of vision (Schön 1996). Undoubtedly, they possess
extremely keen eyesight, especially for moving objects (Cade 1967).
The area between the eye and the beak is a sensitive zone when catching
poisonous or urticantic food. These types of prey vary in composition of
noxious organs or secreting chemical substances, with hard and thick
carapaces.
The Iberian grey shrike, Lanius meridionalis , is a monotypic
species of the family that is geographically restricted to the
Mediterranean region of the Iberian Peninsula and southern France. Given
their tendency to colonize dry open environments, they are scarce at
altitudes above 1000 m.
A remarkable range of prey species are available to them, from
mosquitoes and tiny ants and spiders to tetrapod vertebrates with a body
mass equal to or exceeding their own. Both as an individual and as a
population, the Iberian grey shrike can be very opportunistic due to its
specialization in temporally and spatially limited prey abundance (Hódar
2006).
While insects are the most common prey item of the Iberian grey shrike,
their diet also includes arthropods, lizards, birds, and small mammals
in Spain (Hódar 2006).
The diet varies both regionally and seasonally. Insects dominate in its
French habitats with only few mammals and birds (Lepley 1998) .
Regarding seasonal variations, Hymenopterae are mainly consumed in
autumn, Arachnidae in autumn and winter, Orthopterae in summer and
autumn, and Lepidopterae larvae in winter and spring including by young
birds. Coleopterae were ingested in large numbers throughout the year.
Carabidae were the main prey in winter and Melolonthidae were
particularly important for adults during nestling, as were Cetoniidae
for the chicks (Lepley et al. 2004).
The loggerhead shrike, Lanius ludovicianus , is able to overcome
the toxic defences of a variety of chemically defended invertebrates
such as the grasshopper (Romalea guttata ), the bella moth
(Utetheisa ornatrix ), and the beetle (Lytta polita ) (Yosef
& Whitman 1992, Yosef et al. 1996). The same behaviour has been
observed in the Levantine shrike Lanius excubitor aucheri in
Israel with the highly venomous Orthoptera Poikylocerus bufonius .
A three-day period likely is presumably allowed for detoxification and
subsequent consumption of unsavoury prey (Fuisz & Yosef 2001).
The present study focuses on the particular structure of the loral
plumage of the Iberian grey shrike in southern France, where they feed
on arthropods which they capture primarily with their beak. Our
hypothesis is that the composition of loral bristles is morphologically
highly specialized for this particular type of diet and prey capture
mode, directly affecting the shape, length, and arrangement of feathers
in the area between the eyes and beak.