Each of the three persons- Abdul, Mala, and Chetan saw 1 unique birds each. Therefore, the number of unique birds=3.
Each pair of the three persons saw a bird different to the other. Therefore $$^3C_2$$ =3 birds were seen by a pair of two persons each.
Everyone saw a common bird.
Therefore total birds = 7.
Let the birds which were seen by Abdul, Mala, and Chetan uniquely be shown as A, B and C.
Birds seen by Abdul and Mala, Mala and Chetan, Abdul and Chetan be shown as AM, MC and AC respectively.
Birds seen by all of them be shown as AMC.
Birds seen by Abdul are A, AM, AC and AMC
Birds seen by Mala are M, AM, MC and AMC
Birds seen by Chetan are C, AC, AM and AMC.
It is given that Chetan saw 4 yellow birds. So, C, AC, AM and AMC are yellow birds.
For Abdul, it is given that he saw 2 yellow birds only. So, A and AM are non yellow birds, as he already saw AC and AMC.
For Mala, as he saw 3 yellow birds, and MC and AMC are already among the yellow birds seen by him, and since AM is a non yellow bird. Therefore M has to be a yellow bird.
Therefore, in total, there are 5 Yellow birds- M,C, MC, AC, AMC. And the other two- A and AM are non-yellow birds.