I remember from a recent stream, you mentioned the avian planet being showcased had red seas due to a high amount of cobalt salts (chloride and something else?) in the saltwater. knowing how toxic cobalt can be, I wonder... what kinds of changes does that push onto their biosphere, and what other unfamiliar pressures did life there have to adapt to?
This is biochem so take my words with a grain of salt because it's not my specialty, but cobalt is toxic to life on Earth mostly because we don't naturally encounter it enough for our bodies to have robust measures for regulating it. Heavy metal poisoning occurs because the metal ions enter your body, bind to molecules that are supposed to be doing other stuff, and stick around because there's no processes for getting rid of them. It's like if an automated shipping warehouse started receiving large volumes of empty boxes with no outgoing label. The empty boxes eventually pile up enough that they clog up the warehouse and impede its ability to function.
So, the reason avians and their biosphere aren't poisoned by the unusually high levels of cobalt is the same reason you don't get iron poisoning living an ordinary life on Earth. Their cellular processes are built around the assumption that they will encounter the metal fairly frequently, and deal with it accordingly (and like we do iron, they use it as a respiratory pigment and in a handful of other organic molecules).
As for other changes ehhhhh I am pretty coy with adding big differences to the sophont planets in RttS because the core narrative conceit is that all the aliens can hang out in the same room together without special suits, but avians do have a slightly heavier planet than Earth and a brighter, bluer star than Sol. Their planet is overall hotter and more humid than Earth and avians tend to be more comfortable at higher temperatures than humans.