DIVERGED SWALLOWS

      When I was a boy living in farmland in Lancaster County, Pennsylvania, I was familiar with barn swallows nesting in open-topped, mud-pellet cradles they built themselves on the sides of support beams in a barn.  And when I entered the barn, parent swallows would dive at me and call harshly to drive me away from their eggs or young in those nurseries.     

     Swallows have always fascinated me.  Their swift, swooping flight is entertaining. I have seen six different kinds in the eastern United States over the years, including barn swallows, cliff swallows, bank swallows, rough-winged swallows, purple martins and tree swallows.  And, having diverged from a common ancestor, all swallows, world-wide, have traits in common with each other, including those species in the eastern United States.  They are all built much alike, with streamlined bodies, long, narrow wings and small beaks, with large gapes for ensnaring flying insects, their only food worldwide.  All species speed and sweep briskly across the sky after their flying prey.  They all have tiny legs, which reduces weight.  And all swallows live and nest in open country, where they have room for their powerful, swift flight after prey.  The nesting site of each kind of swallow is what separates them, reducing competition for prey, and producing their many different species throughout the world.  

     Barn swallows enjoy a world-wide distribution.  They probably nested under ceilings in the mouths of caves, where they got away with creating open-top nests of mud pellets.  Today they build their open-cup nurseries in barns and under bridges where they still have a sheltering roof to protect their young from weather and predators.   

     Cliff swallows, however, traditionally nest on cliffs.  These birds build mud-pellet cradles, each with a roof and an entrance low on the outward side to protect the young from weather and predators.  Today, cliff swallows also place their nurseries on the sides of buildings and bridges.  And, barn swallows and cliff swallows today are more abundant than they ever were in the past, because of their increased nesting on buildings and bridges. 

     Bank swallows are well-named because they dig little tunnels into banks of soil to serve as colonial nurseries.  A soil bank may be many miles from a cliff or cave, thus spreading the swallows across the landscape,and making flying insects more abundant for each group of swallows.

     Rough-winged swallows dig little burrows into streambanks, and use the abandoned nesting holes of belted kingfishers in streambanks.  And, today, they also use drainage pipes that conduct water into creeks as nurseries to raise young.  That helps increase their populations, until high water flushes eggs or young out of those pipes, which does not happen every year. 

     Purple martins traditionally raise young in dead, still-standing trees riddled with abandoned woodpecker holes and other cavities.  Today, martins also rear offspring in colonial nest boxes erected in open areas near water, which probably increased their populations greatly.

     Tree swallows traditionally hatch young in deserted woodpecker holes, and other hollows, in trees, near water.  But, today, they also nest in boxes erected for them, or bluebirds, to reproduce in.  They nest either singly, or in small, loose colonies, depending on the number of cavities available.  They also will fight bluebirds, house sparrows and other kinds of small birds for the right to raise offspring in a natural hollow, or a bird box. 

     Tree swallows have several close relatives that diverged from them.  These swallows live in various parts of the Americas, including violet-green swallows in western North America and Mexico, and Bahama tree swallows that nest on islands of the Bahamas and the Caribbean.  These tree swallow cousins are basically the same in body build, and hunting for flying insects.  And they all rear young in tree cavities and bird boxes.  Diversion of species from a common ancestor must be happening all the time.  Nature is always dynamic; constantly changing.

     The diversion of several swallow species from one common ancestor is only one family group that is ever-changing to survive on Earth.  And its that constant changing that helps make nature so intriguing.