SHIFTS IN NESTING SITES

     Purple martins, barn swallows and chimney swifts are adaptable enough to almost completely shift from natural nesting sites in the eastern United States to human-made ones, because those built structures have been available in abundance, and just as protective as natural ones..  Their shift probably increased the populations of each species because there are far more built places to nest in than natural ones.  And, happily, there are more flying insects, particularly flies, for these species to eat in summer in farmland and cities than there were in natural habitats in the past.

     These three kinds of small birds summering in North America to raise progeny are similarly built for life in extended flight.  They all feed on the wing; catching flying insects as they speed and swoop, high and low, across the sky.  There they are entertaining to watch.     

     But these birds don't compete for nesting sites, which spreads them out so they aren't all feeding on the same insect populations  Little colonies of purple martins originally raised young in abandoned woodpecker holes in dead trees and crevices in rock ledges.  But today, most martin pairs rear offspring in erected apartment bird houses and clusters of hollow, hanging gourds.  And, once, I saw a small group of martins nesting in the large letters of a Sunoco sign while I was pumping gas into my car in Virginia.  At the time I thought "that is being adaptable". 

     Barn swallows originally built their mud pellet nests on rock walls in the mouths of caves and on rocky cliffs.  Now they hatch babies in nurseries plastered to support beams in barns and under bridges and boat docks, built places that protect their young quite well from predators and the weather.

     Swifts built twig cradles down the inside of large, hollow, broken-off trees, using their saliva to glue those cradles to the sheer, inside walls of the hollow trees.  But today, they hatch young on twig and saliva platforms glued to the inside walls of chimneys.  As forests were felled for farmland and towns, swifts turned to chimneys as abundant hollow tree substitutes.

     Obviously, these insect-catching birds have benefitted from built structures, and probably are more numerous today than ever in their life histories.  Being adaptable is a key to success.  And, thankfully, many other forms of life on Earth, plants and animals, are equally adaptable.       

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