Extraordinary large brains in tool-using New Caledonian crows (Corvus moneduloides)

A general correlation exists between brain weight and higher cognitive ability in birds and mammals. In birds this relationship is especially evident in corvids. These animals are well-known for their flexible behavior and problem-solving abilities, and have relatively large brains associated with a pallial enlargement. At the behavioral level, New Caledonian crows stand out amongst corvids because of their impressive object manipulation skills both in the wild and in the laboratory. However, nothing is known about the relative size of their brains. Here we show that NC crows have highly encephalised brains relative to most other birds that have been studied. We compared the relative brain size of five NC crows with combined data for four passerine species (7 European carrion crows, 2 European magpies, 3 European jays and 4 domestic sparrows) and found that NC crows had significantly larger brains. A comparison only with the seven carrion crows also revealed significantly larger brains for NC crows. When compared with brain data for 140 avian species from the literature, the NC crow had one of the highest degrees of encephalisation, exceeding that of the 7 other Corvidae in the data set.