Would giving human-like sentience to a bird make the bird too energy expensive to fly?

Question

If hawks or eagles (or any creature that can fly like a hawk) had the same ability to pronounce words as parrots but also had consciousness (what I mean by that is that they can scout and describe the enemy movement to their side), would that extra capability of their brain hinder their flight?

The ways I imagine it would hinder their flight:

1. The brain size would need to be larger to allow them to have speech and higher-than-normal memory.
2. The brain would require more blood adding to the very energy expensive process of flying.

If it does hinder flight, what are ways to make it scientifically possible (without magic)?

Answer 1

We don't know what is necessary for human level intelligence, so let's take the human brain as a starting point, and look at three characteristics:

• weight: our brains weigh about 1.5 kg. As noted by @Slarty, some humans show intelligence with almost half their brain removed, so there's probably some room to maneuver.
• calories: our brains require at least 260 kcal a day To function.
• blood supply: our brains require about 750 millilitres per minute, or 15% of the cardiac output

The largest animal known to be able to fly is the Quetzalcoatlus. Conservative estimates of its weight are around 80 kg, with 250 kg more likely. This puts it in range of a human body, so it makes it a good candidate.

Step one, brain weight. Can we put 1.5 kg of brain into the Quetzalcoatlus' head without breaking stuff? One promising target is the gigantic beak. If we change the intelligent bird's diet (and require them to speak) we may be able to get rid of 90% of the beak and replace that weight by a brain. I can't find the weight of a Quetzalcoatlus skull, but in humans, the entire skeleton is about 15% of our weight, and despite having hollow bones, birds' skeletons have roughly the same weight. With that figure, conservatively estimating the beak to be one twentieth of the skeleton, we get a weight saving of about 0.5kg if we shorten the beak by 90%, if the body weight is 70kg. For less conservative estimates of the body weight, we get closer to 1.5kg.

Next up, calories. For non-passerine birds, the calorie intake per kg is pretty similar to mammals. Assuming 70kg body weight, we get around 2000 kcal, assuming 200kg, we get 5000 kcal. Either way, the 260 kcal required for the brain to operate is a relatively small addition, which can probably be handwaved by making the available food a bit more nutritious (possibly as a result of the intelligence increasing with evolution, as it was for humans).

Finally, blood supply. The neck of the Quetzalcoatl is long, requiring a powerful heart to get enough blood up there. Giraffes have a similar configuration. Their brains weigh half of what a human brain weighs, and they require an 11 kg heart to supply it with blood. Adding 10kg to a 70kg pterosaur might be a deal breaker. In a 200kg beast, there's probably a bit more wiggleroom.

One obvious solution is to shorten the neck. While many large birds have long necks, others, like bustards and condors have short necks, so the reason the Quetzalcoatlus had a long neck may be more to do with feeding than aerodynamics. So, starting with the (likely sizeable) heart the Quetzalcoatlus already had, if we shorten the neck and the beak, we can keep the cardiac output the same, sending more blood to the brain. As a bonus, since the smaller beak puts the center of mass of the skull closer to the end of the neck, less muscle tissue is required in the neck, which also means more blood for the brain.

I'm sure I've missed something, but looking at these considerations, it seems plausible to put a human brain into something ythat flies the way a Quetzalcoatlus did.

Update after comment

What can we do if we limit ourselves to the basic dimensions of a bald eagle? It is estimated that the maximum wing loading (amount of weight per unit of wing surface) for animals is 20 kg/m^2. I can't find the wing surface of a bald eagle but I recon that with a wing span of 2m, a single square meter is reasonable (the wings are about a quarter as deep as they are wide). In other words the bald eagle is much lighter than its maximum weight. Swans have roughly the same wingspan at twice the weight, so this bears out. They also have much more trouble taking off, so at 12kg we are probably approaching the limit of what nature can do with a 2m wingspan.

So, at 6kg for a standard bald eagle, there is plenty of room to fit in a 1.5kg brain and still be able to lift off.

A bald eagle's diet is about 150 kcal, so it would need to more than double its intake to 310. Cooked food and agriculture should get you some of the way there, but you can also give a much richer natural source of nutrition.

The total cardiac output of a pigeon is 200 ml/min at rest, and 1000 ml/min active. Assuming that this scales linearly, the bald eagle's heart produces about 1200ml/min at rest, which we need to almost double to accommodate the new brain (we can ignore the output required to supply the original 12 grams of brain). Heart mass and cardiac output have a linear relation so we need to double the size of the heart, and probably the lungs as well. The heart mass for anything weighing 6kg is no more than a few tens of grams, so that easily fits our weight budget. Lung mass is closer to a few hundred grams, but still easy enough to double without getting into trouble.

Finally, at this size the skull volume is a big problem. We need to up the cranial capacity from 16 to 1000 cubic cm. Note that this seems worse than it is, due to the cubic relation between scale and volume. Quadrupling the length, width and height of the head would be enough. This would change the proportion of the head and body to something like a toucan. Its head would be around 1/10 of its body. It would be difficult to keep this aerodynamic, but shrinking the brain to the minimum required for intelligence, putting much more brain matter in the nervous system, and elongating it into the neck will go a long way.

In short, I guess you'd end up with something that looks like a swan without a neck, and with giant head, but I think it can work.

Answer 2

It's noted in this paper that birds have evolved brains that have a significantly higher density of neurons than mammals, and it's also said the neurons are smaller, which along with less volume of cerebrospinal fluid for a given number of neurons would mean more neurons per unit of brain mass.

With their higher neuronal densities (Fig. 3 A–C), songbird and parrot brains accommodate about twice as many neurons as primate brains of the same mass and two to four times more neurons than rodent brains of equivalent mass (Fig. 1B).

The paper also notes that birds have a higher proportion of their neurons in the pallium, the parts of mammal and bird brains that are responsible for the more complex and flexible forms of cognition (in mammals the pallium is mainly the cerebral cortex, in birds it's just called the pallium but has convergently evolved to play a similar role, see the articles here and here on how it functions similarly to the mammalian cortex). The paper talks about the increased proportion of neurons in birds' telencephalon, which in mammals includes the cortex (pallium) and the basal ganglia (subpallium):

Thus, in contrast to mammals, larger brains of songbirds and parrots contain increasing proportions of neurons in the telencephalon, and correspondingly decreasing proportions of brain neurons in the cerebellum and other brain regions (Fig. 4 C and D). Neuronal densities in the avian pallium exceed those observed in the primate pallium by a factor of 3–4 (Fig. 3A). Hence, the telencephalon houses 38–62% of all brain neurons in songbirds and 53–78% in parrots (Fig. 4C); the pallium houses 33–55% in songbirds and 46–61% in parrots (Fig. 3D and Table S4). This markedly contrasts with the situation found in mammals, in which the pallium accounts for most of total brain volume, but the cerebellum houses a large majority of brain neuron

The paper concludes:

Our finding of greater than primate-like numbers of neurons in the pallium of parrots and songbirds suggests that the large absolute numbers of telencephalic neurons in these two clades provide a means of increasing computational capacity, supporting their advanced behavioral and cognitive complexity, despite their physically smaller brains. Moreover, a short interneuronal distance, the corollary of the extremely high packing densities of their telencephalic neurons, likely results in a high speed of information processing, which may further enhance cognitive abilities of these birds. Thus, the nuclear architecture of the avian brain appears to exhibit more efficient packing of neurons and their interconnections than the layered architecture of the mammalian neocortex.

This paper says the human cerebral cortex has an average mass of 1233 grams, so if we assume a bird with human-like intelligence would have about the same number of neurons in its pallium but at half the mass that would be around 600 grams, and if we assume around 60% of the mass is in the pallium, the whole brain would have a mass of about 1000 grams. (Though it may be that while 60% is close to the maximum in terms of how pallium-dominated a smaller brain could be, with larger brains the proportion of brain mass taken up by the pallium could be even larger--this article notes that 'Although the human cerebral cortex is the largest among mammals in its relative size, at 75.5% (4), 75.7% (5), or even 84.0% (6) of the entire brain mass or volume, other animals, primate and nonprimate, are not far behind: The cerebral cortex represents 73.0% of the entire brain mass in the chimpanzee (7), 74.5% in the horse, and 73.4% in the short-finned whale (3).' So, maybe it wouldn't be too unrealistic to have a bird with human-level intelligence and a brain with a mass of only say 0.7 kg.)

This is quite a bit heavier than any existing brain (this article says Macaws have some of the heaviest brains at around 25 grams), but if you assume a bird with a larger body size than a Macaw perhaps it could fly with a 1 kg brain, especially if it could tuck its head back to rest on the body the way pelicans do:

And this article notes a pelican can weigh up to about 13.6 kg, so perhaps a brain of around 1 kg or a little less wouldn't be too much to make flying impossible for a bird the size of a large pelican. Larger pelicans like the great white pelican have big wingspans, this article says 2.3 - 3.6 meters (7.4 - 11.8 feet), so if you want a bird significantly smaller than that might be difficult (if you want something like an intelligent bird of prey, there was a giant prehistoric eagle in this size range).

Answer 3

A human level of capabilities can be maintained with a significantly smaller brain as can be seen from the many cases of people with a range of abnormal brains leading normal or near normal lives.

So given a very large bird body it is conceivable that a large enough brain could be fitted into the skull to enable quite advanced cognitive functions. How that combination might evolve is another matter.