The nice thing about this picture is that it so clearly shows the white, pasty stuff on the right side of the dropping. That white paste is composed mostly of uric acid, and here's the story about that:
Every second of the day and night, animals are producing waste products inside their bodies. If those waste products are not removed from the body, the animal may become ill or die. One of the most dangerous of all such waste products is the chemical resulting from the normal breakdown of amino acids (which make up protein) and nucleic acids (of which our genetic material is composed). The main waste product formed during this decomposition is ammonia, NH3, which is toxic -- poisonous.
In mammals such as us humans, our waste ammonia is combined with carbon dioxide, CO2, and converted into urea, CO(NH3)2, which is less toxic than ammonia, and which dissolves very easily in water. Our bodies dissolve their urea in water, making urine. In other words, when we go pee, basically what we're doing is getting rid of our urea. If we didn't, that urea would build up in our systems and poison us.
Unfortunately, urinating is fairly wasteful of water. Reptiles and birds have developed a different, less water-wasting process for dealing with their bodies' ammonia. Instead of converting their ammonia to urea, they mostly convert it to uric acid, C5H4N3O3, which is excreted as a white paste or even a dry, white powder. Very little water is wasted here. And that white, pasty C5H4N3O3, or uric acid, is what makes the right side of the chicken dropping white in the picture!
It seems that birds and reptiles have developed an ammonia-disposal system that in many ways is better than our mammalian system. If our bodies could convert our ammonia to pasty uric acid instead of urea, we wouldn't have to drink nearly as much water, nor would we have to go pee nearly as often!