Let m be the mass of the unknown parrot.
111N = (12kg + m)a (equ.1)
Following is the equation for the whole system pf parrots
222N = (12kg + m + 15kg + 20kg)a (equ.2)
Where, a is the acceleration of the four parrots system.
Dividing both the equations, we get
222/111 = (47 + m)/(12 + m)
m = 23kg
To determine the accleration of the system, substitute the value of m in first equation
a = (111N)/(35kg) = 3.2m/s2