Question

Birds insects and reptiles excrete uric acid why?

Answer 1

Because insects and reptiles do not have an well-developed excretory system like ours. As we have developed systems, we excrete more complex compound known as urea. But as I told you that they have a primitive excretory system, they excrete simpler compound (which is the uric acid) and not urea.
I Hope my answer helped you
 

Answer 2

Nitrogenous wastes in the body tend to form toxic ammonia, which must be excreted. Mammals such as humans excrete urea, while birds, reptiles, and some terrestrial invertebrates produce uric acid as waste.

For more information go this link 

https://www.boundless.com/.../nitrogenous-waste-in-birds-and-reptiles-uric-acid-866-12... <----