Most birds have hollow bones with internal struts that make them very strong. (Exceptions include swimming birds, like loons, which have solid bones to help them dive up to 150’ for food.)
Birds also have a smaller total number of bones than mammals. This is because many of their bones have fused together, making the skeleton more rigid. Birds do have more neck vertebrae than many other animals to help them groom their feathers. But overall it takes over 65 chubby chickadees to equal the weight of one medium squirrel.