Because birds use more energy to convert toxins filtered by kidney to uric acid and pop that out to safe water.

Mamals convert it to urea which is then peed out, which looses the body some water, but requires less calorie.

Fish don’t do that at all, they are basically constantly peing out whatever their kidney is filtering, which requires the least amount of energy but a constant supply of water.

ps.: the acid part in uric acid is why bird shit is so nasty to buildings and car paint etc. if it is not cleaned fast enough.

Signal is not Fediverse! Element/Matrix is!