Among the many important aspects of habitat selection that define the lives of water birds, salinity can be one of the most crucial. Birds that exploit food resources in saline areas along the coast or in inland salt lakes require special physiological adaptations of the kidney, cloaca (end of the intestine where digestive materials and urine are mingled) and nasal salt glands. We may recognize the ability of certain birds to tolerate high salinity habitats and use this as a means of predicting where they are found. For example a large rail seen in salt water is likely a clapper, whereas in fresh water it would most probably be a king rail.
I saw an anhinga at our bayside dock recently and was surprised since this species is typically found in freshwater. Of course Lemon Bay can be low in salinity after heavy rains, but in general it is near full strength sea water (32-35 ppt). A relative of the anhinga, the double crested cormorant, on the other hand can live in both high salinity and low salinity environments. Both feed on fish which are caught by swimming them down underwater.
Another type of fish eating diver is the merganser. In SW FL we see wintering red breasted mergansers in salt water habitats and hooded mergansers in fresh water. Yet red breasted mergansers migrate north to breed in Canada and Alaska in inland freshwater areas. This type of switching in habitat salinity by season is also found in other species such as common loons which winter in salt water and breed in northern fresh water lakes.
Our two species of night herons also differ in their preference for saline versus fresh water habitats. Along the SW FL coast, night herons found in salty habitats are generally yellow crowned night herons, which feed mostly on crabs. In freshwater habitats black crowned night herons are predominant and feed on a wider variety of prey including frogs and reptiles. But yellow crowns do occur in low salinity environments along the coast and inland in some localities where a freshwater crustacean, crayfish, may be eaten.
Among the large terns there is a similar dichotomy in salinity preference/tolerance. The royal tern can be seen at Englewood Beach and other coastal areas all year. Whereas the slightly larger Caspian tern is usually found at fresh water areas such as the birding hot spots, Celery Fields or Myakka State Park. But the Caspian can occasionally be found along the coast, especially during migration. In my photo, comparing a single Caspian (with a more massive and redder bill) in the front foreground with a group of orange-billed royals, there is a single smaller sandwich tern (with light bill tip) which is a saltwater species all year.
The fact that birds can be separated into salt tolerant and intolerant species illustrates a common evolutionary theme, that physiological adaptations for coping with salt in the diet and drinking water are a major factor determining speciation. This can be observed in the occurrence of closely related species, one of which is found in saline habitats and one in low salinity habitats. If these were to interbreed, the genes coding for special adaptations for these dichotomous conditions would be muddled and the result would likely be a poorly adapted animal for either habitat. So evolution tends to lead to separation into distinct species each tolerant of saline or freshwater environments and each living in those conditions for which it is best adapted. But the importance of long distance migration to these birds means that some species have adjusted to a switch in habitat salinity between northern latitude breeding and southern latitude non-breeding seasons.