Many insect-eating birds naturally nest in cavities, but hollow trees are hard to find in most urban and suburban neighborhoods. You can easily attract chickadees or wrens to take up residency by providing a nest box that is built to their preferred dimensions for size, entry hole, and distance from the ground. No need for paint and other décor though; the more it looks like a hollow tree, the better. Place a few wood shavings and a swatch of dried moss inside and they'll really feel at home! A perch should never be added, because all cavity-nesting birds are nimble fliers, and a perch only makes it easier for pest birds and predators to invade. If you build it, they will come ... and in some years you may get two consecutive nests, due to the "housing shortage" for these helpful birds.

Bats also use hollow trees to roost in during the summer while raising their young, and to hibernate in winter. If you provide a bat house that is painted black, placed in a warm and sunny location, high above the ground, you may be able to attract them to take up residency in your garden. Mounting the house on a wood post that has a rough texture will make it easy for the bats to access from below.

Ground beetles and other soil-dwelling beneficial insects need homes too. The shelter of decomposing leaves, wood chips, and other coarse-textured organic mulches attracts them to make a home there and begin reproducing in your garden. The larvae of ground beetles will patrol all the little nooks and crannies where slugs hide, and eat their eggs before they hatch and begin marauding through your lettuce patch!