A study found that over the past 50 years, a third of North America's birds have vanished, with grassland birds being the hardest hit, declining by more than 50%. The study combined data from 13 different bird counts and millions of radar sweeps to estimate the population decline. The loss of birds is attributed to various factors, including habitat destruction and expansion of farms. The decline is considered a massive change in the natural system, and it was previously unknown due to the difficulty of counting bird populations.