"Time for another obscure word [characteristic] in the annals of vertebrate palaeontology and here is one that ties together birds and pterosaurs, if only in a nomenclatural sense. For those that do not know, both pterosaurs and birds have hollowed out, pneumatic bones which in life were filled with air sacs that were extensions of the lungs. However, this obviously could potentially weaken the bones and make them vulnerable to being broken and given the kinds of high forces that many of them would have to deal with (like the bones of the wing or legs for flight and landing respectively) you want to keep them strong.
Evolution has evolved an elegant way around the conflict here – keeping things hollow (and thus light) but strong with some biological scaffolding. The trabeculae are therefore the various small and often intricate little webs and buttresses and spars of bones that populate the insides of various bird and pterosaur bones, providing strength and support to the bone with the minimum of extra mass."
"Although limited by the availability of different ontogenetic stages within single taxa, these studies provide important baseline information about pterosaur osteohistology and led to the deduction that Late Cretaceous pterodactyloids, with wingspans of 3–11 m, such as Montanazhdarcho, Pteranodon and Quetzalcoatlus had ‘typical dinosaurian and even typical bird-like bone growth’ (de Ricqle`s et al. 2000; Padian et al. 2004). These researchers further suggested that some smaller basal pterosaurs of the Triassic and Jurassic (with wingspans up to 1.5 m), especially Rhamphorhynchus, appear to have grown more like smaller birds (de Ricqle`s et al. 2000; Padian et al. 2004). The current study represents the first comprehensive assessment of osteohistological changes during ontogeny within a single pterosaur taxon."
Cross-section of a bird wing bone, borrowed from http://platospond.com/WatsonsBlog/wp-content/uploads/2009/02/image_sci_animal0291.jpg.