They are giving us the parent's phenotype and genotype:
P1: Yellow, heterozygous
P2: Red, recessive homozygous
Let's assign the letter Y to the yellow allele and y to the red allele.
P1: Yy
P2:yy
Then the Punnett's square would be as follows:
This means 50% of the offspring would be heterozygous yellow birds, and the other 50% would be recessive homozygous red birds.
So, if there are 4 fledglings, 2 would be red and the other 2 would be yellow.