On a Summer visit to our Florida island retreat we had an unusual visitor to our small backyard bird bath (see photo). An adult black crowned night heron (BCNH) was standing in the bath and trying not very successfully to drink the shallow fresh water. As a general rule yellow crowned night herons (YCNH) are found primarily in salt water areas such as our yard, and BCNH are found mainly in fresh water. As is the case for many so-called ecological rules, there are exceptions probably related to the availability of food and fresh water. YCNH are specialists in eating crustaceans which generally restricts them to salt water areas; however where crayfish are abundant they can occur in inland sites. For example at Miller Park in downtown Winston Salem, NC, there is a breeding colony of YCNH, far from the coast. At our island location in Florida we only see BCNH because our neighbor feeds them and offers fresh water to drink. On Assateague Island (eastern shore of VA) there are BCNH predominantly on one side of the road where there is an artificially created freshwater impoundment, and YCNH mainly on the salt water side of the road. Marine birds possess a special salt gland above the eye that enables them to drink sea water which the kidney is unable to handle, since sea water has about three times the salt concentration of blood. This explains why BCNH which lack salt glands can only survive in coastal areas if there is fresh drinking water, and why YCNH (which presumably have salt glands) never visit our water bath. Marine reptiles have several types of salt glands of similar function but with different origins and locations.
Now while adult night herons are quite distinctive in coloration, the young are another matter entirely and considerable arguments have raged among birders about identifying immature night herons. I have provided photos of young BCNH and YCNH to compare- notice that the bill of the YCNH is thicker and all dark, whereas the bill of the BCNH is partially light-colored. There are some differences in plumage, neck and leg length, which are not always obvious . But this distinction is one of the most contentious among birders, right up there with arguments about sharp-shinned and Cooper’s hawks.
So while it may not have occurred to you to decide the most likely identification of a bird based on the salinity of the habitat, this provides a very useful clue for identification. If you begin to look at not only animals but plants also, you will find a large number that are often physically separated by their tolerance of low and high salinities and/or their abilties to compete with species that differ in their physiological tolerances to salinity.
Bill Dunson, Englewood, FL & Galax, VA