Burton, RF. 2008. The scaling of eye size in adult birds: Relationship to brain, head and body size. Vision Research 48: 2345-2351.

Following the lecture on light signal reception, we went into more depth on the physiology of vision and optimal eye design. The discussion of constraints on eye development (for example, that compound eyes can only compress so many cells into one area of the head) inspired me to research another type of constraint on eyes—that of the brain. My understanding of the human brain is that a very large percentage of it is devoted to the transmission and translation of images we see, and I was curious to see if any research had been performed on comparing the relative selective pressure of eye size to brain size. This was the primary topic of consideration in the Burton article, which postulated that birds should have large eyes for body size overall given that vision is the primary sensory modality for birds. However, the author noted that the space in typical bird skulls for the eyes is poorly defined, partially because the eye and brain are contiguous. Burton thus planned to compare eye size to body AND brain size to see which comparison would show a stronger correlation.

Birds to be evaluated included 38 species divided into three groups based on general lifestyle (falconiforms, parrots, and song birds for example). Brains and bodies of sample birds were weighed and sizes of the brain and eye cavities were measured using plasticine spheres that were required to fit “comfortably in the sockets without contact with the bone.” Sizes were treated as logarithms in statistical analyses to equalize variances, and regressions of eye versus brain size and eye versus body weight were performed.

Results from the allometric study were mixed, but seemed to favor the comparison of brain size as predictor of eye size, rather than body weight. However, scaling among the different lifestyle subgroups revealed various trends. Non-passerine birds tended to have eye size that was predictable by brain size, but this trend was confounded in parrots and passerine species. Parrots in particular had very large brain to eye ratios, which seemed to follow the convention that they are among the most intelligent of birds, however visually impaired. This was also true for passerine species, as passerines include the family Corvidae, the well-studied jays, crows, and ravens. These are another highly intelligent family that seems to have evolved a large brain without large eyes to supply it with information. The corvid brain to eye comparison, however, was not considered statistically significantly small enough to buck the general trend of larger eyes per larger brains

Burton found only one family of birds that appeared to have a consistent eye size to body size relationship—the owls. Thus he argued that, in general, it is possible that selection forces the eye and brain to compete for head space in a way that draws a stronger correlation than that of overall body mass to head size. Although eyes in this study were always treated as spheroid, we learned in the February 26 lecture that many animals do not have perfectly round eyes (and the same is true for many humans with vision problems!). I thought it might be compelling to include some measure of eye or retinal surface area in the measure of eye size to compare to brain size. This would require preservation of data beyond what was apparently available for the scope of the Burton paper, but may be meaningful in future studies of allometry.