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.