Bluetooth bird feeder - Pigeons and Magpies

My brother was bought a Bluetooth bird feeder for his birthday and absolutely loves it!! 

Only problem is Pigeons and Magpies are dominating it, scaring off the smaller birds. 

Can anyone recommend any tips as to how he can deter the larger birds and is there a bracket or front people can recommend which can be added to allow the smaller birds access whilst ensuring the camera and focus is not impacted?

Any guidance or advice would be great thank you :)