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Abstract.
The rook (Corvus frugilegus) is widely distributed in the Western Palaearctic and is generally regarded as a pest species of agriculture and at airfields. In this study we describe a schematic eye model for the rook. Digital photographs of the intact enucleated eye gave the gross dimensions. The lens and the cornea of the rook's schematic eye were found to have an almost equal refractive power of Fl 59.53 D and Fc 62.63 D. The rook's schematic eye has an axial length of 14.50mm and a total power of F 108.86 D. The eye shape is similar to the 'flat' eye type. The calculated f-number^sup 2^ value of 3.63 indicates that the rook's eye is well adapted to high light levels and is not suitable for night vision.
Key words: Corvus frugilegus, optical design, lens, cornea, pest
Introduction
Crows (Corvidae) are well known to possess exceptional intelligence including episodic memory and tool use (e.g. Emery & Clayton 2004, Kenward et al. 2005, Seed et al. 2006). They also show a highly evolved skill at caching food items for retrieval and consumption in the future (Coombs 1978, Goodwin 1986, Healy & Krebs 1992). This ability includes learning the geometric relationships among landmarks that are crucial to the relocation of cached food items (Kamil & Jones 2000, Kamil & Cheng 2001). It would appear therefore, that the visual system in crows plays a crucial role in the location and retrieval of stored food items. In addition their visual systems are essential inter alia for vigilance against predators and sexual selection. However, relatively little research appears to have been undertaken into the corvid eye and the visual physiology of crows.
The first stage of vision involves the manipulation of light by refraction to present an accurate image of the world at the retina. This is achieved by two principal refractive structures, the cornea and the lens. This image formation can be modelled in a "schematic eye", which is a quantitative description of the optical properties of these principal refractive components (Martin 1983).
Rooks form flocks and feed mostly in open grassland habitats (Goodwin 1986, Cramp & Perrins 1994, Madge & Burn 1999) where they may be regarded as a pest of agriculture (Dunnet & Patterson...