December has 31 days, January has 31 days and February has 28 days for a total of 31+31+28 = <<31+31+28=90>>90 days
He feeds them 1/2 cup in the morning and 1/2 cup in the afternoon for a total of 1/2+1/2 = <<1/2+1/2=1>>1 cup per day
If he feeds them 1 cup per day for 90 days then he will need 1*90 = <<1*90=90>>90 cups of birdseed
#### 90