Passenger pigeons( 旅鸽 ) once flew over much of the United States in unbelievable numbers. Written accounts from the 18th and 19th centuries described flocks( 群 ) so large that they darkened the sky for hours. It was calculated that when its population reached its highest point,there were more than 3 billion passenger pigeons — a number equal to 24 to 40 percent of the total bird population in the United States,making it perhaps the most abundant bird in the world.Even as late as 1870 when their numbers had already become smaller,a flock believed to be 1 mile wide and 320 miles (about 515 kilometers)long was seen near Cincinnati. Sadly, the abundance of passenger pigeons may have been their undoing . Where the birds were most abundant, people believed there was an ever-lasting supply and killed them by the thousands. Commercial hunters attracted them to small clearings with grain, waited until pigeons had settled to feed,then threw large nets over them,taking hundreds at a time.The birds were shipped to large cities and sold in restaurants. The underlined word “ undoing ” probably refers to the pigeons’ .