Story, first published: Galaxy Science Fiction, Oct. 1965
Simak tells the story about a time traveler who goes back and discovers aliens herding up dinosaurs for food, which accounts for the dinosaurs' sudden disappearance as a species. The ending of the story chillingly implies that our human population might have grown enough for the aliens to consider coming back.
Ewald, Robert J.: When the Fires Burn High and the Wind is from the North, p.135
In this story, Simak made aliens responsible for the sudden disappearance of the dinosaurs - they were rounding them up and slaughtering them for food - and as soon as our population grows large enough to attract their attention, the aliens could come back.
Ewald, Robert J.: When the Fires Burn High and the Wind is from the North, p.107