There are many reasons why we wish to encourage more forms of life to live in our gardens in greater numbers. Most importantly they interact to more effectively prevent any pests from increasing in numbers than we can by direct means. Usually overlooked is the fact that they contribute much fertility in their daily droppings and dead bodies. They are also a great pleasure to behold in their daily goings on.
Although we often remember to provide food as an attractant; be it berries for the birds, pollen or nectar rich flowers for the insects or just catfood for the hedgehog we often forget where they are to live. The biggest problem for most creatures is finding somewhere to call home; somewhere to reproduce or hibernate in. Most of our gardens are too manicured and leave no good places suitable for nests that remain undisturbed for long enough. In particular birds suffer from a shortage of nest holes in trees as we too promptly remove any dead wood.
Now of course any bird box is probably useful to some bird or another; a wren or a robin will often make use of the most bizarre object or place to put their nests. Old teapots and kettles tied inside evergreens, spout down for drainage, will often be moved into by these small friends. However most other birds are much more selective and some will fail to breed at all if they cannot find just the right spot.
Naturalists have observed bird are clever to avoid some not so obvious dangers, for example most birds prefer the entrance hole not to face the sun as a blast of strong rays can burn little fledglings. But they cannot be expected to appreciate the drainage properties or thermal behaviour of man made materials. Sad cases occur such as the drowned nest in my old crash helmet hung in a leaking shed. Experimenters in Victorian times tried mass produced tin birdboxes which inadvertently alternately roasted and chilled the birds. Others have made, and still make, terra cotta boxes which may be better than tin but are probably far too cold and subject to condensation. Wood has proven to be the only really suitable material.
And there is a hidden danger to the usual wooden nest box even so. Most bird boxes are made rectangular, something not common in nature. Although the birds can build nests within these boxes there may be a tendency for these to later deform to a squarer shape. In itself this is not a problem but it may interfere with the regular rotation of the clutch of eggs. It is easy to see how an egg, or four, could be wedged into the corners and not get the regular turning and warmth required. Most conventional nest boxes I put up I now pre-form into a bowl shape inside with some plaster.
A nineteenth century German, Count Berlepsch, noted the shortage of natural nest sites in his managed woodlands and started investigating. He found the majority of useful, mostly insectivorous, birds preferred to nest in a discarded woodpecker's hole. These were obviously in relatively short supply. He had standardised replica woodpecker holes made in logs turned on a lathe and had these hung in his woods by the hundreds. He found the majority were utilised within a year or so. His research showed that there was a tremendous shortage of natural holes which were the limiting factor for small bird populations as he achieved a very high density of successfully bird nested boxes running at hundreds per acre.
Berlepsch's original nest box would be somewhat expensive to make nowadays as it requires a large lathe, and billets of wood are not so cheap as then. I have been experimenting with a papier mache' alternative that I hope should be as successful though I am slow to make any number. Anyway the critical bit is the inside shape, see insert, this is bowl shaped at the bottom, rough ribbed sides to allow egress, an exit hole sloping downwards and roof space above this. I made a wooden former to exactly the inside shape; on this I build up the next shape in layers of newspaper soaked in flour & water glue. (A former could be made from a plastic lemonade bottle built up with plaster or papier mache'.) (To make parting easier I tried a balloon, well something very similar, stretched over the former before adding the first layer, this would not follow the valleys so I use cling wrap which allows the dried nest to be prised off the former with some care.) Once I have built the newspaper layer up into a thick coat, enough to be rigid or about a quarter inch, I leave it to dry. I then cut down one side and prise off the shaped inner shell. This I then tape back together and build up to a uniform cylinder with a layer of shredded newspaper, egg box carton and flour glue mache'. Once this has dried I wind it in corrugated cardboard to make a thicker 'log'. The outer layer and lid are covered in recycled plastic bags or roofing felt or underseal and then bark chunks, sawdust and wood shaving scraps are glued on with waterproof glue. (The entrance hole is made by cutting it in once the roll is complete but before the bark is applied.)
But of course not all birds want actual boxes; swallows and house martins want inaccessible crevices and nooks to make their nests in and are also hard to entice away from traditional sites. They can however be helped by creating a pond with a muddy edge they can come to and by leaving out hair on shrubs nearby for them to use for their nest building materials. Fly catchers may use a box but often prefer to build on top of one rather than in it; they seem to want ledges much like swallows. The songbirds in particular prefer to nest where they can start one easily by adding twigs to a naturally forking framework of branches. More of these can be created during pruning by cutting back shrubs so they grow many stems from a fork. Or alternatively supple branches can be tied or woven together during winter to create similar forking supports. Ideally in either case the fork becomes obscured as the growth leafs up around it.
Naturally owls and other large birds may be difficult to attract or accommodate to the average garden but fortunately the majority of the most useful garden birds are the small ones which are easy to cater for. And similarly useful in many ways are the bats. These fly at night as do gnats and mosquitoes, and every orchard caterpillar pest I can think of is a night flying moth. Thus bats are very good friends. They're hard to accommodate with winter hibernation quarters unless you have a cave or similar. They may use summer breeding boxes if you provide them; flat open ended wooden boxes the size of a really big telephone directory fastened high up on warm walls apparently do nicely.
Hedgehogs are best provided for when a new shed or garden construction is made. They need a large dry space they can pack with leaves and hay. The inside of a loose bonfire appears inviting too often; luckily once when I had a hedgehog move into a bonfire heap it was spotted, the situation formalised (viz. bonfire became hedgehog sanctuary) and the heap waterproofed. Now I convert piles of refuse I would previously have burned into layers of cladding for hedgehog shelters made of wooden boxes buried deep underneath. The piles can be smartened up with a few nasturtiums rambling over them each summer.
I don't rightly know what a frog likes to overwinter in or under, I suspect much the same as toads who are said to like a large old pot hidden under a pile of rocks. Many people have kept toads in their coldframes and greenhouses as they eat so many pests but few encourage newts, yet I have found these at night in the middle of my vegetable beds so I guess they wander far. I've often uncovered newts under slabs of concrete or big logs so these indicated the sorts of nests I needed to make for all these invaluable friends. Next to my pond and pools I have built piles of logs and others of rocks, bricks, old bottles and stones, full of cracks and crevices of all sizes and incorporating old drainage pipes and earthenware pots as ready made dens.
Bumble bees, beetles, and unfortunately slugs and snails also like piles with nooks and crannies but these can be on a smaller scale. Of course we do not wish to encourage the molluscs but at least we can trap them if we know where they sleep. Thus the ideal pile is a half dozen saucers, or a couple of flowerpots inside one another or similar. The beetles are good friends and can be left unmolested when the other are evicted. I have a little flowerpot stack at the shady end of most of my raised beds. In mulched areas chunky rough cut bark on top of another permanent mulch makes quite a good place for beetles but unluckily the birds know too.
Spiders also benefit from a chunky bark mulch; many species are ground hunters and spin no webs, these thrive and multiply on the areas given permanent straw mulches. Of course the web spiders proliferate in undisturbed long grass bunds but need no homes as such. In the greenhouse though I tie strings to encourage web formation in the most useful places, and not across the path, and provide hollow tubes of dried plant stem as wee homes. It is amazing how the spiders take to these.
Hollow stems also make good homes for ladybirds, and some predatory wasps, lacewings and hoverflies that overwinter. I tie bundles of foot long stems of such as teasel and then hang these inside evergreen shrubs and hedges. Others are stuffed inside plastic bottles stuck on sticks amongst the soft fruits and vegetables. Ladybirds seem to prefer old sites that have been previously used (successful ones in other words) so collecting up the odd ladybird and introducing it to a new home encourages more of them to move in much sooner.
And of course we can provide homes for honey bees. They are the most rewarding of insect friends as the crop of honey and wax is well worth the efforts and expenses involved if you have the time to look after them. In cold windy years your own bees may manage to pollinate your crops, such as apricots and peaches, which are close nearby when all other trees remain unvisited.
The marvellous thing about providing many different sorts of nests is that most of them don't cost a penny to make and within a short time you can see them being moved into. The more nests you provide the more different creatures come and they make the garden so much more interesting. Indeed it's only when the garden becomes full of little friends that it really seems to come alive; and I'm sure the plants appreciate the company too!