How Bird Brains Endow Corvids and Parrots with Primate-Like Intelligence

 

                                               Comparing the Brains of a Monkey and a Raven

                                                          A Diagram of the Avian Brain

"Bird brain" is a derogatory term.  But it shouldn't be.

Yes, bird brains are smaller than primate brains.  And bird brains don't seem to have a mammalian cerebral cortex or prefrontal cortex, which are largely responsible for complex cognition among mammals.  So we might not expect much from a walnut-sized bird brain.

But many of the studies of animal intelligence have shown that some birds--particularly, corvids (crows, jays, ravens, and jackdaws) and large parrots--have a complex cognitive psychology comparable to that of primates (Emery and Clayton 2004).  They use and manufacture tools.  They engage in causal reasoning.  They show flexible learning strategies.  They have some social intelligence.  They can use their imagination to form cognitive maps of their world and run simulations of future situations.

This complex cognitive functioning in corvids is controlled by the avian pallium, which corresponds to the mammalian cerebral cortex.  The pallium is the layers of grey and white matter that cover the upper surface of the cerebrum in vertebrates.  In mammals, the cortical part of the pallium forms the cerebreal cortex.  Within the avian pallium, the highest cognitive functions are controlled by the high numbers of associative neurons in the mesopallium and nidopallium, which correspond to the prefrontal region of primates (Herculano-Houzel 2020).

Using the isotropic fractionator method, Herculano-Houzel and her colleagues have shown that birds have large numbers of neurons in their whole brain, in their pallium, and in their mesopallium and nidopallium.  Among the songbirds studied, with brain mass ranging from 0.36 to 14.13 g, the total number of neurons in the brain ranges from 136 million to 2.17 billion.  In the parrots studied, brain mass ranged from 1.15 to 20.73 g, while the numbers of neurons ranged from 227 million to 3.14 billion. By comparison, a rhesus monkey brain weighing 70 g has about 1.7 billion neurons. This illustrates how neurons are more densely packed in the brains of birds than in primate brains--just as neurons are more densely packed in primate brains than in the brains of other mammals, which Herculano-Houzel calls the "primate advantage."  So here we see the "avian advantage" (Olkowicz et al. 2016).

                                                              A Blue-and-Yellow Macaw

                                                                A Bonnet Macaque Monkey

The avian pallial neurons of corvids and parrots significantly outnumber the pallial neurons of primate brains of the same mass.  For example, a raven pallium weighs about 10.20 g, and it has about 1.2 billion pallial neurons, while a capuchin monkey pallium weighing about 39.18 g has about 1.1 billion neurons.  A blue-and-yellow macaw with a pallium weighing about 14.38 g has about 1.9 neurons, while a bonnet macaque pallium weighing about 70 g has about 1.7 billion neurons.

The associative pallial neurons in the avian mesopallium and nidopallium are thought to be the functional equivalent to the mammalian prefrontal cortex, which drives flexible and complex cognitive performance.  So if we find similar or even higher numbers of neurons in associative pallial areas in corvids and parrots as in some primate species, that would help explain why these birds have primate-like intelligence.  And, indeed, some research has shown that corvid mesopallium and nidopallium combined can have between 200 and 300 million neurons per hemisphere, which is more than the estimated 68 million neurons in the prefrontal cortical region of the rhesus monkey.

But even though these large numbers of neurons in the bird brain are a major factor in explaining the complex intelligence of corvids and parrot, we should keep in mind that "complex cognitive functions obviously also depend on many other variables like cellular morphology, connectivity patterns, neurochemical properties, and cognition-related regulatory genetic sequences" (Strockens, et al. 2022, 1602).

What we see here is one of the major themes of my writing--the emergent evolution of animal minds in the brain:  differences in degree in the mental capacity of the brain can produce differences in kind when they pass over a critical threshold in the size and complexity of the brain.  That's why the approximately 16 billion neurons in the human cerebral cortex helps to explain why human beings have some mental capacities--such as language and symbolic abstraction--that other animals do not have at all.

REFERENCES

Emery, Nathan J., and Nicola S. Clayton. 2004. "The Mentality of Crows: Convergent Evolution of Intelligence in Corvids and Apes." Science 306:1903-1907.

Herculano-Houzel, Suzana. 2020. "Birds Do Have a Brain Cortex--And Think." Science 369:1567-1568.

Olkowicz, Seweryn, et al. 2016. "Birds Have Primate-Like Numbers of Neurons in the Forebrain." Proceedings of the National Academy of Sciences 113:7255-7260.

Strockens, Felix, et al. 2022. "High Associative Neuron Numbers Could Drive Cognitive Performance in Corvid Species." The Journal of Comparative Neurology 530:1588-1605.