Advice requested for planting under bird box

Hello,  

I wondered if anyone could advise me on the best shrubs and plants to put under a bird tit box,  

It is the first year that they've used it and it has unfortunately coincided with our getting two new young cats,  were doing our best to keep them away as the chicks will be fledging soon but one fell out earlier and we've now had to lock the cats in. 

Any suggestions would be greatly appreciated 

Regards Teresa