Materials and methods
In order to examine the structure of rictal bristles in the Iberian grey shrike in southern France and evaluate their potential protective function against venomous prey, we first studied larders with potentially venomous prey species in order to assess the danger these prey may pose to the shrike’s eyes after being caught and carried in the beak. In a second step, we examined zoologically naturalized and recovered Iberian grey shrike in natural history museums and combined them with high-resolution photographs that show the structure and orientation of the rictal bristles in natura during arthropod capture.
Study of noxious arthropods found on the Iberian Grey Shrike larders.
Since the Iberian grey shrike mainly impales its prey in winter, we focussed on the period between December 2018 and the end of February 2019 to record a total of 341 prey items on a vineyard in southern France (43.810070 N, 4.201536 E) (Labouyrie 2020). Eight additional large prey were photographed in November 2021 at another location twelve kilometres away (43.791740 N, 4.04955 E). As smaller prey (<10mm) are eaten directly and do not appear to be impaled, we mostly found large arthropod prey items, including species with spiny parts or spines with chemical secretions that could get into the eyes during handling on the floor or in larders.
• Examination of specimens in museum collections.
Vibrissae are a highly specialized type of feather in which the spine is relatively stiff, more tapered, and free of beards for most of its length. Essentially, vibrissae are functionally simplified contour feathers that are found almost exclusively on birds’ heads, and they are still clearly visible on the specimens stored in the collections.
We measured the length of the rictal bristles and beaks (from the base of the culmen to the tip of the upper mandible) on all available specimens from the Mediterranean plains in southern France (outside the Iberian Peninsula) registered in the collections of the Natural History Museum of Nimes and the National Natural History Museum of Paris (both France) (Table 1).
Rictal bristles form a cluster of four to five hairs at the base of the beak (Figure 1) and we measured the longest of these hairs with a digital calliper. The length of the beak was measured at the base of the culmen.
• Photographic study in natura.
We took high-resolution images of the head of Iberian grey shrikes from short distances of seven to eight meters, using an ornithological blind and a progressive approach to minimize the disturbance of the birds.