Presumably your assignment will be evaluated, not on its effectiveness at scaring away birds, but whether the system can detect something like a PIR sensor being triggered and the ability to generate a signal for an ultrasonic transducer.
Select the components:
Arduino, PIR, transducer.
Design code, initially to operate each device, as a test, then join it all together.
Are there any special requirements mentioned in the assignment such as for how long, after the initial detection of the “bird”, the transducer should operate or if the project intended to be battery powered etc.
A more realistic use of the built device would be to scare away cats.