sadly round here for the last 5years I have not so much as seen a squashed Hedgehog let alone a live one. The birds seem to like fat balls and now seem to have gone off the nuts and seeds.
Most birds have already got their nest sites organised so just keep an eye out for them taking a look at potential new homes, but it might be next year so don't be disheartened. There's lots of food around for birds at this time of year so they may not be interested in feeders just now, but once it gets to autumn you'll find they start coming in. It also depends what birds are in your area so do a little research and take a look at what visits so that you can tailor the type of feed you use. It can take a while for birds to find your hospitality as they may have regular sites elsewhere which they go to first. I've been in this house just over a year and they're getting to know the garden better now - it just takes a little time
I feed fat ball, seed, nuts, mealworms and any fruit I have that is passed its best. I dont ground feed but the other birds seem to drop enough to keep the larger birds happy. I have consciously planted shrubs and perenials that attract wildlife. I do tidy the garden for the winter but I do have a log pile and have created wildlife 'homes' with housebricks behind some of the shrubs. I have a birdbath and a half barrel and also try and leave some water in a shallow dish on the floor. The birds line up to feed and we get lots of insects, frogs and toads, I havent seen any hedgehogs. We do get the odd cats visiting the garden and I have to say that occasionally I see evidence of a bird kill, although that could be the sparrow hawks that drop in occasionally.
They always peck at the holes, it's a habit they have but not sure of the reason. They will eventually enlarge the hole, but oddly that means larger birds can get in, which makes the box unsuitable! (some things about nature are not easy to work out!). It doesn't mean the holes are not the correct size to start with though.
One can only guess your boxes are not quite right for them for some other reason. I face the hole to the north and put them on fairly long sticks well away from cats in trees or bushes. Avoid busy areas such as those used by lots of birds to get to feeders, they won't choose a box where they have to compete all the time with other birds It's often wise to make your own from the dimensions given out by wildlife trusts etc, they usually work as they have been tried and tested many times. The plans for the ones I've done are here:
Mike Allen wrote (see)
Mike hasn'tread all the replies. However to answer the original question. Add some glycerine to the water. It perfectly safe.
Please don't add glycerine to birdbaths - it is a low level toxin and because it is sweet if the birds drink the water it will raise their blood sugar level and this could kill them. It could also damage the water-proofing and insulation properties of the bird's feathers which could also kill them.
Only clean pure water in birdbaths please.