A study was done on the birds that use these rivers. The birds were identified and counted once during the summer and once during the fall, both in the morning and in the evening. As can be seen by the figure, the number of different kinds of birds seen in each study area was nearly the same (11 species along the wastewater study area and 10.5 along the control), but the total number of birds present was greatly increased along the wastewater study area. Two different groups of birds account for this difference. One group are the flocking birds associated with agricultural fields and development, such as blackbirds. Several large flocks were counted during one of the counts on the wastewater site. The other group were the ones feeding on fish, such as egrets, herons, and kingfishers. The nutrients in the wastewater appear to cause algae to grow rapidly, which then becomes the food source for algae-eating fish, such as the tilapia. At certain times of the year the river almost becomes a "river of fish", an abundant source of food for the fish-eating birds, thus explaining the large number of herons and egrets found at the wastewater site.