Why some bird brains are larger than others

How does brain size and design influence the survival chances of a species? A large brain may contribute to an individual’s success irrespective of its detailed composition. I have studied the size and shape of cerebella in birds and looked for links between the bird’s cerebellar design, brain size and behavior. My results indicate that the cerebellum in large-brained birds does not scale uniformly, but occurs in two designs. Crows, parrots and woodpeckers show an enlargement of the cerebellar trigeminal and visual parts, while owls show an enlargement of vestibular and tail somatosensory cerebellar regions, likely related to their specialization as nocturnal raptors. The enlargement of specific cerebellar regions in crows, parrots and woodpeckers may be related to their repertoire of visually guided goal-directed beak behavior. This specialization may lead to an increased active exploration and perception of the physical world, much as primates use of their hands to explore their environment. The parallel specialization seen in some birds and primates may point to the influence of a similar neuronal machine in shaping selection during phylogeny. The cerebellum is a highly conserved part of the brain present in most vertebrates [1], well suited for a comparative study of size and design. The cerebellum in birds, as in mammals, consists of a strongly folded, thin sheet of gray matter, located dorsally to the brainstem. In birds, it largely consists of a single narrow strip that varies in different species in the anteroposterior extension, which corresponds to the cerebellar length. The cerebellum of birds is commonly subdivided into ten groups of folds termed lobuli [2]. Both variability and regularity are evident in the lobular pattern of the bird cerebella. To quantify these structural varieties and relate them to functional or phylogenetic differences, a principal component analysis was performed on the residuals of the lobuli length, obtained from a double-logarithmic regression of lobuli length against body size (see Supplemental Data on-line for further details). Generally, birds within a family (Figure 1) tended to score similarly on the two principal components (PCs). The variability in the principal plane is dominated by variability between bird families (one-way ANOVA with bird family as factor: F(23, 24) = 4.59, p < 0.001). The group of birds that scored highly on the first PC consisted of crows, parrots and woodpeckers. In contrast, the nocturnal owls had a different growth pattern and scored highly on the second PC. Both PCs together explained 66% of the total variance (first PC, 44%; second PC, 22%). The cerebellar growth of the first group is based on the enlargement of Larsell’s lobuli IV and VI–IX, while in the owl group lobuli I–II and X increase in size (Figure 2A,B). The difference in enlargement of these two groups of lobuli in the two groups of birds, i.e., crows, parrots and woodpeckers versus owls was statistically significant (Figure 2C). The following groups of cerebellar lobuli can be related to functional subdivisions through their afferents: somatosensory (tail, III–IV; leg, IV–V; wing, IV–VIa; head, V–VII and VIIIb–IXa) [3,4], visual (VIc–IXc) [5,6], auditory (VII–VIII) and vestibular (IXd–X) [7]. My analysis indicates that, in the diurnal bird group (crows, parrots and woodpeckers), the cerebellar regions that receive visual and trigeminal inputs show the