The perpetuation of any species requires reproduction. If we want to help wildlife, we need to help assure that we'll have wildlife year after year. Nesting sites are a critical part of any backyard wildlife habitat.
Nesting boxes for cavity-nesting birds are easily built from pine or cedar. These boxes imitate natural cavities in dead trees. It's easy to find many plans and styles on the internet. Look for plans that include a swing-away wall or roof so that you can clean the box out at the end of nesting season. Different birds require boxes and entry holes of different sizes, so spend some time observing the birds that you have in your area and build nesting boxes suitable for them. Houses can be built for birds ranging from tiny wrens to screech owls. Leave the inside of the box rough, or score some groves on the inside under the entrance hole to help baby birds climb out. It's also a good idea to leave it unpainted so that it blends in with the background. Birds tend not to choose brightly-painted birdhouses that are too conspicuous. In addition to birds, squirrels will often use nesting boxes designed just for them.
Predators can be a real problem around nesting boxes. Raccoons, squirrels, cats, and rats will all prey on nestlings if they can reach them. A predator guard, a cylinder of wire mesh fixed to the front of the box, helps exclude predators.
Many birds prefer nesting platforms. These birds normally nest in the fork of a tree, but will often use a well-designed platform for nest construction. Other birds prefer to build nests in trees or thick shrubs. Just planting a tree in your yard if you have none already will vastly increase the value of your yard to wildlife.
Larger, permanent provide reproductive opportunities for amphibians and aquatic insects. Be sure to leave some debris in the bottom of the pond, and provide heavy rocks or drain tiles on the bottom to give protection from predators. If a pond is large enough and natural enough, it might even attract ducks.
Rock piles, brush piles, and log piles, discussed in the article on Cover, also provide nesting sites for some species of birds and for small mammals. Some insects and spiders lay their eggs in these piles, and they in turn provide food for many of the larger animals that use piles for nesting and hiding.
Butterflies seek out larval food plants for laying their eggs. Because so many people whisk out the pesticides at the first sign of caterpillar activity in their garden, reproductive opportunities for butterflies are limited in the suburbs. People don't often think that the green caterpillar that they despise is the infant form of the butterfly they admire -- or even if they do, they may still protest, "But I don't want those awful things eating my plants!" To assist butterflies, do some research on the internet to find out what butterflies are native to your area. Find out what kind of plants their larvae need, and plant these in a quiet, protected corner of their yard. Many will be native plants that need little care. If your larval garden is out of sight, perhaps your neighbors will never notice that you're providing a feast for caterpillars, and you can help boost the butterfly population in your neighborhood.
Food, water, cover, and nesting sites -- provide all four, and you'll soon see a lot more wildlife in your yard.
The entire Gardening for Wildlife series:
Gardening for Wildlife: The 4 Element
Gardening for Wildlife: Food
Gardening for Wildlife: Water
Gardening for Wildlife: Cover
Gardening for Wildlife: Nesting Sites