Merlins in / Rough-legs out

     Yesterday I saw another merlin, a beautiful adult female hawk, in cropland between New Holland and Ephrata in Lancaster County, Pennsylvania.  I've been seeing more merlins in that farm country in recent winters, and almost no rough-legged hawks, though there was a time when rough-legs were fairly common in that human-made, open habitat in winter.
     Merlins raise young in trees where the boreal forests of Canada and Alaska become thin as they mingle with the Arctic tundra.  That's why merlins winter in open country in the United States.  
     Rough-legs hatch offspring on cliffs on the tundra of Canada and Alaska.  They, too, winter in open country.  So why are merlins wintering more in Lancaster County farmland in recent years and rough-legs are declining in that some, local habitat? 
     I noticed as more pairs of red-tailed hawks have been raising young in our farmland and more red-tails are wintering here, rough-leg numbers have declined here in winter.  Red-tails are bigger, tougher hawks than rough-legs, and red-tails defend winter hunting territories.  Meanwhile, only limited numbers of mice and rats can live in cultivated fields that are harvested to the ground in autumn, leaving little food and shelter for those rodents.  My theory is the stronger red-tails chased out most of the rough-legs to have the limited prey animals to themselves.  Those Arctic hawks couldn't stand up to the competition and winter somewhere else.
     Rough-legged hawks have smaller, weaker feet than red-tails.  The former species is built for catching mice, of which there are few in our intensely cultivated farmland because cultivation often disturbs the soil, making it hard for mice and their food plants to become established in the fields.  But the more robust red-tails are adapted to catching mice, rats, squirrels and other kinds of larger animals in local cropland.  Red-tails are better adapted to our agricultural areas, and they take advantage of that fact to out-compete rough-legs.
     Merlins, on the other hand, specialize in catching small birds, which are abundant in our fields in winter, especially horned larks and house sparrows.  Merlins don't compete much with red-tails for food, so the red-tails seldom bother merlins.  And merlins are too fast in flight to be threatened by the soaring red-tails anyway.  Therefore, merlins are better suited for and do well in our cropland while rough-legs don't.  Merlins win.  Rough-legs are in direct competition with increasing numbers of red-tails, and can not defend themselves against the latter species of hawk.  Rough-legs lose.
     Nature is never stagnant, but forever changing, due to varying conditions.  And each species of life is successful when it adapts to a niche that is relatively free of competition.