This is a different approach to a bird scarer.
When the batteries hold enough charge (not a full charge) the energy supplied by the small but efficient solar panel, is fed to a low power, high torque planetary geared motor.
Through a series of gears this energy is used to slowly lift a shaft against the force of a spring attached to a small bellows attached to a simple horn. When the shaft reaches its maximum height the shaft suddenly drops with the stored energy of the spring, compressing the bellows and making the horn blast out a short intense burst of sound.
This should be enough to deter any birds within 15-20 meter radius of the bird scarer. Not a totally impressive range, but the design is only 570mm tall with the ground spike and 260mm tall without it.
The design allows for an inexpensive, easy to use, low maintenance unit that would be very effective if multiple units were deployed, they need not all be activated at the same time, although this could be accomplished with the addition of a sound activated switch, but would be possibly more effective if they activated in a random pattern to discourage the possibility of the birds learning any pattern within the timing of the sounds.
At the end of the needed period the units can easily be stored, ready for the next field.