Solution for #31

Three birds were seen by one person each, three were seen by each unique pair (Abel-Mabel, Abel-Caleb, and Mabel-Caleb), and one was seen by all three. So seven birds were seen in all, and each person saw a total of four. Hence, all of the birds Caleb saw were yellow. These four birds are: (1) the one Caleb saw alone, (2) the one Caleb saw with Abel, (3) the one Caleb saw with Mabel, and (4) the one all three saw together. This accounts for both of the yellow birds Abel saw, and two of the three yellow birds Mabel saw. The third yellow bird Mabel saw could not have been the one Abel and Mabel saw together, because Abel only saw two yellow birds; so the third yellow bird Mabel saw must have been the one she saw alone.

So five yellow birds were seen (the one Mabel saw, the one Caleb saw, the one Abel and Caleb saw, the one Mabel and Caleb saw, and the one all three saw), and two non-yellow birds were seen (the one Abel saw and the one Abel and Mabel saw) by the group.