Larger animals are more prone to extinction due in part to smaller numbers. The asteroid that impacted 65 million years ago killed off the larger mammals and dinosaurs but spared some of the smaller ones. The smaller niches were refilled by both mammals and Theropods (as birds), but there is a paucity of large bird species. Is there a reason mammals were better at refilling these large-animal niches?
Birds have to feed and guard there eggs for 30-40 days, against the elements and against predators, whereas mammals can bear larger offspring in exposed environments. That's especially an advantage for swimming mammal megafauna, because the birds can't lay eggs in water, and also for predator-intensive environments and for forests.
Birds lack hands and teeth and four sets of claws for manipulating leaves and prey and defense. Birds can only have a hooked bill to peel up prey, and less sophisticated molars for herbivory. The sheer range of mammal teeth shapes appears to be an advantage compared to beaks.
Quadrupeds have some advantage for running and turning without falling over, whereas birds mostly have long necks which are a bit of an easy target for horns and claws and teeth.
It's interesting to think about a pack of wolves attacking an ostritch, and a pack of predator ostritches attacking a springbok... The wolves and the springbok have a wider range of attacks and defenses to use for survival.