Diverging and Converging Merganser Ducks

     Three kinds of merganser ducks in North America are examples of related species of life diverging into different niches from a common ancestor, which reduces competition for space and food with their relatives.  Mergansers radiated from other species of ducks to exploit a food source (small fish) different from that of other duck species.  They don't have the broad, shoveling beaks of other duck species, but developed thin, serrated bills, adapted for seizing fish and other aquatic creatures.  And, at the same time, mergansers developed like, and converged with, unrelated, fish-catching species (loons and grebes) because the water habitat they share shaped all of them alike. Mergansers, loons and grebes all have boat-like bodies, webbed feet and beaks shaped for catching small fish. 
     The three kinds of mergansers in North America diverged from a shared merganser ancestor to catch fish in different water niches in winter, thus reducing competition with each other for that food.  Common mergansers generally get their finny prey in the deeper parts of large bodies of fresh water, such as rivers and larger, human-made impoundments.  Hooded mergansers usually snare their food from fresh-water creeks and ponds.  And red-breasted mergansers catch fish mostly from brackish or salt water in estuaries and back waters off the oceans. 
     In winter, scores and hundreds of common mergansers raft on the Susquehanna and Delaware Rivers and human-made, fresh-water lakes in southeastern Pennsylvania where I live, as throughout much of the United States.  There they are associates of American goldeneyes, buffleheads, scaups and other kinds of ducks, none of which catch fish.
     Drake common mergansers have dark backs, green heads, white flanks and red bills.  Hens of this species are camouflaged, with gray body feathering and brown heads with ragged crests.
     Hooded mergansers generally gather in little groups on the edges of smaller bodies of water and swamps, often in or bordering woodlands.  They usually hide under tree branches hanging over the shallows, as do wood ducks. 
     Male hooded mergansers are handsome with black backs and heads, white chests, chestnut flanks and with a horizontal, white bar on each side of their hoods on their heads and a vertical, white stripe in front of each wing.  The white on their hoods gets larger when they raise those hoods in excitement. 
     Female hooded mergansers are attractive, too, in a plain, camouflaged way.  Their plumages are light, tawny-gray with a darker, disheveled crest. 
     Red-breasted mergansers winter mostly along the seacoast that they share with a variety of sea ducks, including scoters and other species.  They uncommonly migrate north through southeastern Pennsylvania in March.  A few land on larger bodies of water in this area to catch fish before moving on.  Red breast males have green heads, each with a crest, dark backs and light-gray flanks.  Females have gray bodies and brown heads with ragged crests.
     Mergansers demonstrate how life radiates from common ancestors to take advantage of different niches to have space and food with limited competition with their relatives.  And each habitat makes the unrelated species living in it similar, adapted to survive in it.