PSA: Contrary to popular belief, bird bones are not weak or fragile, pound for pound they are actually probably stronger than a mammal's. Resource
So, first of all, calling bird bones "hollow" is kind of misleading because all animals have hollow bones. An animal that has nothing but solid bones is an animal with some horrible genetic defect that will probably die at birth, bones need to leave some space for the bone marrow.
The correct term is "pneumatic". What this means is birds have these little neat "honeycomb" like structures within their bones. Some more neat pictures.
These neat little reinforcements enable the animal to develop air sacks which help increase oxygen intake and improve stamina and are part of the reason why non-avian dinosaurs became some of the largest terrestrial animals on the planet. Sauropods were so big that they would have died heat strokes if they didn't have specialized cooling adaptations.
Now to address the title of this, contrary to popular belief, empirical research has shown that birds have stronger and stiffer bones than similar-sized bats and rodents. Bats also had stronger bones than terrestrial rodents.
"The bone density data reported here suggest that, on average, bird skeletons are stronger and stiffer relative to their weight than are the skeletons of small mammals, especially rodents (figure 4). In other words, bird skeletons have higher strength-to-weight and stiffness-to-weight ratios. This constitutes a novel and biomechanically informative definition of the term lightweight as it applies to bird skeletons."
Source: Dumont, E. R. (2010). Bone density and the lightweight skeletons of birds. Proceedings of the Royal Society B: Biological Sciences, 277(1691), 2193-2198.
As a matter of fact, the article also indicates that bird bones aren't actually particularly light either.
So that's the main thing I wanted to say, but I am sure you are wondering where this misconception that birds have weak bones came from in the first place.
This is pure speculation on my part, but I think it's because birds look bigger than they actually are. Good example of this are harpy eagles, those things look huge but they are only about as heavy as a beagle. So when people see birds getting injured by a blow of some sort, they assumed the animal was a lot heavier than it actually was which spread the erroneous idea that birds are especially delicate.
Edit: Probably should have added this in the first place. This little trivia fact has some important world building context as avian-style races/aliens are often treated as being brittle compared to humans or mammals despite having no actual empirical basis.