The expression “birdbrain” should be interpreted more as a compliment than an insult. Getting and staying airborne is a highly complex activity requiring finely-tuned coordination and sensory perception, which is partly why birds fill us with wonder and pilots are generally held in high esteem.
Birds accordingly have large brains with swollen cerebral hemispheres, more sophisticated than those of most creatures apart from some mammals. These big brains not only allow birds to stay aloft, but also to perform some impressive mental feats: Caledonian crows and many other birds make and use complex tools, and some resourceful blue tits learnt how to open the foil tops once used in milk bottles, then taught others across the UK this useful skill.
Dinosaurs, on the other hand, have long been caricatured as lumbering behemoths with tiny brains: children learn that Stegosaurus was as big as a car but had the brain the size of a walnut.
This is a bit of an exaggeration: nevertheless, the peach-sized brain in the elephant-sized Stegosaurus, or the apple-sized brain in the much larger Apatosaurus, were very unimpressive. Certainly, many dinosaurs, especially quadrapedal herbivores such as these, had relatively small, reptile-like brains.
But it is now increasingly clear that other dinosaurs were much brainier.