This story is not one of the strong ones. It is too contrived. The visiting aliens are evolved from birds, the starship cruises in a planetary atmosphere, the Earth stuffers a catastrophe that leaves just one little sliver habitable, these aliens are roaming the galaxy manually -- no robot help. It's a whole lot of contrivance just to get to the twist at the end. One young boy survives with a crow pet, and one young girl. Raging hormones in a boy who doesn't remember ever seeing a girl.
Yup... too contrived to keep my interest this time. Moving on after page five...