Terron eggs cannot grow or hatch with light exposure. If the egg is exposed to more than 30 seconds of light, the creature inside will die instantly. This is why these birds are so rarely found in Ark. Stay away from the light!
Terron sleep during the day and come out only at night (similar to that of a bat). And boy, they sure do soar the skies at night. And to humans, this is a good thing. An ancient legend tells that if a human were to look upon a Terron's illuminated body, they would become terrified and die of fear. No one has dared test this legend. During the day, Calla rule the skies, but by night, Terron. This is why stargazing is feared in Ark.