Interspecific allometry of the brain and brain regions in parrots (psittaciformes): comparisons with other birds and primates.

Despite significant progress in understanding the evolution of the mammalian brain, relatively little is known of the patterns of evolutionary change in the avian brain. In particular, statements regarding which avian taxa have relatively larger brains and brain regions are based on small sample sizes and statistical analyses are generally lacking. We tested whether psittaciforms (parrots, cockatoos and lorikeets) have larger brains and forebrains than other birds using both conventional and phylogenetically based methods. In addition, we compared the psittaciforms to primates to determine if cognitive similarities between the two groups were reflected by similarities in brain and telencephalic volumes. Overall, psittaciforms have relatively larger brains and telencephala than most other non-passerine orders. No significant difference in relative brain or telencephalic volume was detected between psittaciforms and passerines. Comparisons of other brain region sizes between psittaciforms and other birds, however, exhibited conflicting results depending upon whether body mass or a brain volume remainder (total brain volume - brain region volume) was used as a scaling variable. When compared to primates, psittaciforms possessed similar relative brain and telencephalic volumes. The only exception to this was that in some analyses psittaciforms had significantly larger telencephala than primates of similar brain volume. The results therefore provide empirical evidence for previous claims that psittaciforms possess relatively large brains and telencephala. Despite the variability in the results, it is clear that psittaciforms tend to possess large brains and telencephala relative to non-passerines and are similar to primates in this regard. Although it could be suggested that this reflects the advanced cognitive abilities of psittaciforms, similar studies performed in corvids and other avian taxa will be required before this claim can be made with any certainty.