Ythrians

Philippe Rochefort watches a recorded lecture about the current enemy. Since, despite reading Poul Anderson's descriptions, I never retain any mental image of an Ythrian, let us paraphrase:

warm blooded;
feathered;
flying;
not birds;
young born viviparously;
four and a half months gestation;
females cannot fly far with a heavy pregnancy;
lips and teeth, not beaks (despite the attached image);
not mammals;
no hair or milk;
infants fed by regurgitation;
walking awkwardly on feet that have grown from wings;
standing on hands to lift wings;
flapping of wings pumps oxygen through gill-like antlibranchs into bloodstream, enabling a body heavy enough for intelligence to fly in terrestroid conditions;
large energy intake needed;
some sweet fruits eaten;
otherwise carnivorous;
therefore, living in small groups, each defending a wide territory;
evolved not from reptiloids but from amphibians;
primitive land animals retained a kind of gill;
small swamp-dwellers climbed into trees and developed a membrane for gliding;
membrane became wings as gills became superchargers;
internal water-hoarding system;
light but strong bones;
tiny, helpless cubs cling to either parent with elaborate digits retained on wings;
young also evade predators by climbing trees;
feet seized prey and manipulated objects;
rapid metabolism of flight prevents young from being born with undeveloped nervous systems;
parents cooperate to care for and carry young;
sexual equality;
females ovulate only once per Ythrian year (half Terran) and not for two years after giving birth;
Ythrians sexually active only at these times;
grief causes ovulation;
occasional females able to ovulate at will were killed, now shunned;
drought forced ornithoids onto savannahs where they evolved from carrion eaters into hunters;
feet became hands and made tools;
elbow claws became feet;
wings became convertible to legs;
hunters strike from above with spears, arrows or axes;
less need for cooperation than among proto-men;
beaked hawk-like uhoths used like dogs;
Stone Age ended not by agriculture but by herding and domestication of animals, encouraging invention of wheels for land vehicles;
agriculture an ancillary, providing fodder;
no cities;
flight, so no need for crowding;
sedentary centers for mining etc small with floating populations and wing-clipped slaves, the latter now being replaced by machines.

As on Diomedes, Poul Anderson has imagined not only a non-humanoid intelligent species but also its evolution.