A certain amount of the muddy colour can be attributed to the different colours of food we eat. Like mixing all the paint colours together, the result is a dull brown. But, much bigger factors for humans' brown poop are bilirubin and bile. Bilirubin is a yellow substance found in the liver, the product of the breakdown of old red blood cells. Bile is dark brown or green and is produced by the liver to help digest fats. Both of these substances are secreted into the small intestine during digestion, and slowly make their way into poop, bringing with them a dark brown hue.
Bird poop, on the other hand, is not brown but white. That is because—unlike mammals—birds don't pee! Instead, where mammals get rid of waste ammonia in our urine in the form of urea, birds eliminate their waste ammonia in the form of uric acid, which is a white paste. That way they don't carry extra water weight around. The brown dot you usually see in the middle of a bird dropping is the actual poop, and it is usually brown.