Where is the gas-exchange surface in the bird respiratory system, i. e. where does oxygen enter the blood and carbon dioxide leave the blood

the lungs

Explanation:

The avian respiratory system consists of three sets of organs that perform respiration: lungs (where gas exchange occurs), anterior air sacs and posterior air sacs. The lungs have static structures with surfaces that enable gas exchange, and air sacs that enable the unidirectional flow of air through the lungs. Birds have a very efficient respiratory system that obtains oxygen when they breathe in and breathe out. The inhaled air reaches the primary bronchus where enters to the lungs for gas exchange and also fills the air sacs.

the oldest known insects are believed to live around 400 millions ago as suggested by the fossil records.   some plants having woody stems also evolved during this time.

after the evolution of insects the first four legged species or the tetrapods evolved in shallow freshwater habitats from the intermediate species approximately 397 million years ago. the tetrapods went on conquering the land giving rise to amphibians, reptiles, mammals and birds.

the major split in tetrapods first occurred 340 million years ago branching off from amphibians.

363 million years ago when the carboniferous period began the insects started roaming on land and taking flights, following which the four limbed tetrapods adapted in the terrestrial habitat.

explanation:

the sun shines white and black light to our planet. the white light has all the colors in it. in our upper atmosphere, we have dust particles and oxygen molecules. the white light meaning all of the colors in a rainbow shines through the upper levels of the atmopshere and the blue light scatters across the entire planet. that is why our sky is blue.