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Bigger bird brains grow at the expense of the parents

Despite the fact that the term “birdbrain” is often used to denote a person who is silly or stupid, some birds actually have quite large brains. In fact, the brains of birds are often as large as, or even larger than those of mammals of similar body size. However, there are distinct differences in the brain-to-body ratio found in altricial and precocial bird species, and this has led scientists to question whether brain size may be linked to the degree of development of chicks at the time of hatching. 

Precocial birds, such as ostriches, ducks, chickens and gamebirds commonly produce large clutches of eggs that hatch into chicks covered in down and ready to walk about and forage immediately. Adults of these bird species have relatively small brains compared to the size of their bodies. 

In contrast, altricial birds, such as songbirds, produce smaller clutches of eggs that hatch into naked, blind and helpless chicks. Parents of these chicks have to provision them with food as they develop feathers, mature and fledge from the nest. Even after fledging, the chicks need to be fed for several weeks until they are independent. But once they are adults, these birds have relatively large brains and are often considered to be more intelligent.

Brains are very important organs and one might think that the bigger the brain, the better. Large brains support numerous cognitive adaptations and enable individuals to solve problems and make decisions that are innovative. However, large brains come with costs. They are slow to mature and expensive in terms of energy. In fact, they require a large and constant supply of energy in order to develop and function at full potential. 

Biologists are aware that the helpless altricial chicks of large-brained birds would certainly not be able to supply themselves with enough food while their underdeveloped brains matured. They would face an impossible hurdle – unless their parents helped them by supplying food. This led researchers to question whether having a large brain is linked to the provision of energy by parent birds to their offspring. 

A team of scientists led by behavioral and evolutionary biologist Michael Griesser of the University of Konstanz has now compared the relative brain size in adults of 1,176 different bird species, with the extent of parental care. The researchers quantified how much energy parents invest in raising their offspring, including the egg size, clutch size and duration of post-hatching feeding and care. The results of the study have been published in the Proceedings of the National Academy of Sciences (PNAS).

According to Griesser, precocial species of birds, such as ostriches, chickens, geese and ducks, have large clutches and large eggs. “But that’s already the end of the parental energy input.” Since these chicks leave the nest shortly after hatching, and can walk around and peck up their own food, they do not require much more input of energy from the parents. 

“In contrast, altricial birds, such as sparrows, parrots and crows, hatch underdeveloped. Their growing brains have a different neuronal structure compared to those of precocial species: brains of altricial young only need to be capable of supporting body growth, while brains of precocial species have to fully function right after birth,” explained Griesser. Furthermore, young altricial birds need to be fed both in the nest and often after fledging, and thus it takes several weeks, at times even months, until they are nutritionally independent from their parents.

The researchers made use of published datasets in the literature in order to study the relationship between avian brain size and parental provision of energy to the offspring. They conclude that the critical factor for the size of the brain is how much energy the parents invest in the growth of their young, via egg size and active feeding of the young after hatching. 

“Large brains, in particular, grow at the expense of the parents,” said Griesser. The chicks of birds considered to be clever, such as parrots, crows, owls and other raptors, require a lot of energy for their brain growth. Corvids (members of the crow family) have the largest brains relative to their body size. Without an input of energy from the parents, the chicks of these large-brained species would be unable to grow to maturity.

Previous research has shown that large-brained species of birds are better able to find food sources and remember them, even after some time has elapsed. They are also more successful at living in urbanized habitats or settling into novel environments. 

The results of this study might also explain differences in brain size in other animals. The parental energy provided by fish, amphibians and reptiles is largely limited to producing the eggs. These animals have much smaller brain-to-body ratios than birds or mammals. Thus, long provisioning by parents seems to be a general prerequisite for the evolution of large brains, as is also evident in humans. 

“There is ample evidence suggesting that parental provisioning and brain size evolved together and that only lineages with extended parental provisioning have the ability to evolve larger brains,” concludes Griesser.

By Alison Bosman, Staff Writer

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