Shape from shading in starlings (Sturnus vulgaris).

One important visual property of most natural objects is that they have curved continuous surfaces that differentially reflect light. As a consequence, the patterns of lighting, shading, and shadow reflected from surfaces contain useful information about objects’ shape and location in the environment. These types of lighting information make essential contributions to the human perception of objects, depth, and scene layout (Gibson, 1950, 1979; Mingolla & Todd, 1986; Norman, Todd, & Orban, 2004; Ramachandran, 1988). They are also critically involved with many algorithms for machine vision and object recognition (Horn & Brooks, 1989; Zhang, Tsai, Cryer, & Shah, 1999), and the graphic visualization and portrayal of realistic scenes (Kniss, Premoze, Hansen, Shirley, & McPherson, 2003). Given their reliance on vision for important activities such as flight and foraging, birds might also be sensitive to the shape information given by differential shading, shadows, and specular highlights (shine). While prior studies with birds have employed a variety of methods, making generalization difficult, they suggest that some of these lighting cues are used in both natural and laboratory settings (Cavoto & Cook, 2006; Hershberger, 1970; Hess, 1950; Reid & Spetch, 1998; Rowland, Cuthill, Harvey, Speed, & Ruxton, 2008; Young, Peissig, Wasserman, & Biederman, 2001). Cook, Qadri, Kieres, and Commons-Miller (2012) recently investigated in pigeons the contributions of shading to the perception of 3D smooth, curved surfaces. Their results indicated that pigeons readily use relative patterns of shading to identify the concavity or convexity of different shapes in a visual scene. The pigeons transferred a discrimination of shaded surface direction to novel perspectives of the scene, across changes in illumination, specularity, and lighting direction, and to novel surface shapes. When subsequently tested with videos in which the scene’s lighting continuously changed, the pigeons continued to recognize shapes independent of the nature of the lighting. Together, these results suggest that illumination-based information makes an important contribution when pigeons visually derive shape and depth-based information about objects and their spatial relations in a scene. Pigeons have been an important focus species for studies of complex perception and learning in comparative cognition (Cook, 2001a; Honig & Fetterman, 1992; Spetch & Friedman, 2006; Zeigler & Bischof, 1993). This development is more historical accident than design, as the selection of this columbiform species for early operant studies was driven primarily by considerations of pragmatism, husbandry, and availability. The intervening years of intensive study have resulted in the pigeon visual and cognitive system being by far the best understood of any avian species. It remains an open question, however, as to whether the visual cognition of pigeons is representative of the over 9,000 birds within this class of animal (Bock & Farrand, 1980). Unfortunately, there is a regrettable lack of comparable research in any other bird species. Such comparative information is crucial to understanding the operation and function of vision in the behavior of birds, the evolution of this important modality, and its interaction with the processing of features in the natural visual environment. A better understanding of vision in birds generally would advance our understanding of the overarching principles by which different visual pattern recognition systems represent the world (Marr, 1982). Our goal here was to advance the broader comparative investigation of visual processing by examining visual cognition in a passerine using the same approaches developed with pigeons. Despite being a highly successful order and extensively studied for other aspects of bird behavior, knowledge about the visual and cognitive mechanisms in passerines is decidedly limited. For this purpose, we chose to test starlings, another highly proliferative species of bird like the pigeon. European starlings (Sturnus vulgaris) are a medium-sized passerine species. They are visual, diurnal, ground-feeding birds that forage for invertebrates by probing the upper soil surface or turf with their bills (Feare, 1984). Starlings have been an increasingly popular species for study with widespread interest in their foraging and auditory behavior (Page, Hulse, & Cynx, 1989). Passeriformes and columbiformes diverged during the Cretaceous period between 70 and 120 million years ago according to genetic estimates (Brown, Rest, Garcia-Moreno, Sorenson, & Mindell, 2008; Chojnowski, Kimball, & Braun, 2008). A multivariate analysis of the brain compositions of different birds has suggested that since diverging, passeriformes and columbiformes have evolved dissimilar cerebrotypes, with different relative proportions of cerebellum, brainstem, and telencephalic regions, including the relative area devoted to vision (Iwaniuk & Hurd, 2005). At least one study has suggested that starlings and pigeons may differ in their lateralization of different visual functions (Templeton & Gonzalez, 2004). Here we investigated the perception of illumination-based visual features by this passerine species. This was done to address the serious gap in knowledge about visual cognition across different orders of birds and to understand where avian visual processing is situated within the larger set of visual pattern recognition systems. We evaluated a working hypothesis that illumination-based cues for shape perception are a fundamental visual property for birds despite peripheral and neuroanatomic differences, and thus shape-from-shading mechanisms are common and consistent across different orders. We tested the starlings using a two-alternative forced-choice perching task to examine their processing of the same shading-based scene stimuli tested with pigeons (Cook et al., 2012). If illumination-based information is salient to the perception of depth and shape information across birds, this hypothesis predicts that the starlings should react similarly to the pigeons across the manipulations tested here.