How did the grants test their hypothesis that differences in birds’ songs can keep different species of finches from breeding with one another? how di

The offspring had a smaller range of beak depths­­from smallest to largest­­than their parents (black bars).The overall beak depths of the 1978 offspring population are larger the original 1976 population.In 1978, the offspring population is larger than the initial 1976 population before the drought.The finches that survived the drought (black bars) had offspring with much larger beaks. 
n both droughts, small­beaked medium ground finches had the highest survival rates.In both droughts, large­beaked medium ground finches had the highest survival rates.Both droughts events provided strong natural selection on medium ground finch populations.Both droughts resulted in changes to food sources that favored the survival of some medium ground finches overothers.