Question

how could we determine that birds are very closely related to reptiles​

Answer 1

Explanation:

Feather provided insulation to dinosaurs in cold weather. Later in the evolutionary process, feathers were used for flight in birds. Because dinosaurs were reptiles, it means that birds are closely related to reptiles.

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Answer 2

ʙɪʀᴅꜱ ᴀʀᴇ ᴄʟᴏꜱᴇʟy ʀᴇʟᴀᴛᴇᴅ ᴛᴏ ʀᴇᴩᴛɪʟᴇꜱ ʙᴇᴄᴀᴜꜱᴇ ʙᴏᴛʜ ʜᴀᴠɪɴɢ ᴛʜᴇ ꜱɪᴍɪʟᴀʀ ᴄʜᴀʀᴀᴄᴛᴇʀɪꜱᴛɪᴄꜱ. ʙᴏᴛʜ ʙɪʀᴅꜱ ᴀɴᴅ ʀᴇᴩᴛɪʟᴇꜱ ʟᴀyꜱ ᴇɢɢꜱ, ᴄᴏɴᴛᴀɪɴꜱ ʀʙᴄ. ʙᴏᴛʜ ʙɪʀᴅꜱ ᴀɴᴅ ʀᴇᴩᴛɪʟᴇꜱ ʙʀᴇᴀᴛʜɪɴɢ ᴩʀᴏᴄᴇꜱꜱ ɪꜱ ꜱᴀᴍᴇ ᴛʜᴀᴛ ɪꜱ ᴛʜᴇy ʙʀᴇᴀᴛʜᴇ ᴠɪᴀ ʟᴜɴɢꜱ.