As the name suggests this egret follows large grazing animals such as cows or horses feeding on insects (grasshoppers, beetles etc) these animals  harbour or disturb. You may also see these birds on close the runway at wallblake airport feeding on insects blown out of the grass by the passing planes.
It is believed that the  cattle egret originated in Africa and as the Sahara dust does every year, it crossed the Atlantic Ocean,  making its way from South America northward in the 19^th and 20^th Century.