Sparrow Nest Box

Last year I made a sparrow nest box. Sited the box in the hawthorn Hedge at the bottom of my garden. To date the sparrows have shown no interest , seen them sitting on the roof of the box along with Blackbirds. The entrance holes are drilled at 35mm diameter. Do you think I should put perches in front of the holes ? There is a fair size flock of sparrows that habitate the hedge , making plenty of noise. That is until I refill the feeders when they all go really quiet. Trev