Final answer:
Arguments against the tree-down gliding model include the model's insufficiency in explaining the evolutionary adaptations for flapping flight, the irrelevance of certain bird features for simple gliding, and behaviors of flightless birds that may suggest terrestrial origins.
Step-by-step explanation:
Arguments against the tree-down gliding model for the origin of bird flight often focus on the idea that this theory does not fully explain the evolutionary pressures and adaptations necessary for powered flight. One argument is that the arboreal hypothesis, which suggests that flight evolved from feathered dinosaurs gliding between trees, does not account for the muscular and skeletal changes required for the flapping flight of modern birds. Another argument is that the presence of features such as hollow or pneumatic bones, which are critical for reducing body weight and aiding in flight, would have been unnecessary for simple gliding from tree to tree. Flightless birds, such as the ostrich, exhibit behaviors like spreading their wings while running, which could suggest a running or terrestrial start to flight rather than a gliding one.
Lastly, the fossil record shows a variety of bird-like dinosaurs with different levels of flight capability, and it's hard to distinguish which of these might have led to the fully capable fliers we see today. Taken together, these factors pose questions about the sufficiency of the tree-down gliding model in explaining how the complex behavior of powered flight in birds could have evolved.