Bird Houses and Cavity Nesters

BIRD HOUSES AND CAVITY-NESTERS

by Jon Friedman

Introduction
All birds need a few certainties in their lives to survive. Critical among them are food, water, shelter and habitat. Not all birds share the same requirements for living. Water is a common necessity that all birds need daily. But the foods they eat and the habitats they live in can differ widely. All birds require shelter as well. Where birds spend their nights and where and how they nest differs significantly by species.

Photo by Doris Evans

Some pelagic birds like the albatrosses actually sleep on the wing as they glide through the night above the seas. Some birds scratch out a small depression in the dirt and sleep there. Cavity nesters sleep in saguaros, tree holes, holes in telephone poles, etc. Others simply pick out a safe place away from predators, strong winds, rain, etc. These birds may use tree branches, eaves of houses, architectural crevices and ledges, and even the spaces afforded them in the channel lettering of signage for commercial businesses. And, some birds use the homes that bird lovers provide for them in their backyards.

Humans have been providing housing and nesting sites for birds for hundreds of years. In North America, before the appearance of European colonists, Native Americans had long been using gourds to provide homes for birds. Europeans have a long history of using bird houses, too. In our contemporary times, with our advantage of scientific knowledge and technical information, it is safe to say that more birds are offered backyard housing now than ever before. However, all houses are not equal.

Materials – cedar is best
In order for a bird, or a family of birds, to actually use a house it must meet certain criteria. There is no one bird house that all birds can use. Several factors will determine whether or not birds can, or will, use any particular house. The materials it is constructed from could play a role. The best material for bird houses is still wood. Cedar is clearly the best wood, for its resistance to weather and moisture, its longevity, its insulating factors, and the fact that insects will not bore into cedar and its oil will deter bees and wasps from setting up shop. In recent years and with the advent of “thinking green,” recycled plastics are proving more popular year after year. The material is thick enough to provide good insulating factors for both heat and cold. Birdhouses made from pine, fir, spruce, and especially plywood should be avoided for a variety of reasons and plywood is composed primarily from wood shavings, adhesives, and nasty chemicals, such as formaldehyde. Wooden bird houses are perhaps most attractive to birds when they are unadorned and left to age and weather naturally. Most natural woods will take on a slightly textured surface and the wood will turn to a silvery-gray color – much like that of old barn wood.

Other materials
Metal bird houses may be OK in northern tier states but offer no insulating value for heat in our desert region. In metal houses, eggs can slow cook and babies can be fried. Gourds are still used for bird housing. We’ve sold Manny Garcia’s folk art bird house gourds for years. I think it is better to use natural gourds rather than plastic gourds, especially in our hot climate. They can be natural or painted. Be sure to always use a non-toxic, water-based paint. In our region, always use white or light-colored paints that reflect sun and heat better than darker colors that tend to absorb heat. Most experts will agree that aged and weathered birdhouses will appeal to birds more than new, brightly painted ones.

Dimensions and direction
The actual dimensions, or specifications, are critically important – especially if you are interested in attracting certain species of birds. The size of the entrance/exit hole, the height of the hole above the interior floor, and the overall dimensions (height, width, and depth) must be correct for the intended species. If you want to provide housing for more than one species, you will need to consider putting up multiple houses of differing dimensions. Most bluebird houses fill the bill as the best “generic” houses in that their dimensions are good for many similar-sized songbirds. The height above ground is very important for some species. The direction the house faces could also play a role for some species. In our region, it’s a good rule of thumb to position the opening of the house to the north, which is ideal, or the east, which is next best. This is especially true if the house will be in the direct afternoon summer sun. If the house is located in a shady area, then the direction the house faces is less important.

Ventilation and drainage
All birdhouses need both ventilation and drainage. Ventilation is important to help control high temperatures inside the house. There’s usually some accommodation for air circulation with vents either in the highest corners or in slits near the top of the house. Drainage holes are required in all houses, even in Arizona. Drainage is usually accomplished with open floor corners or holes drilled into the bottom of the house. Slanted roofs enable rain to quickly run off.

Maintenance requirements
All birdhouses need to have a clean-out panel for annual or semi-annual maintenance. This can vary greatly with each model design. Sometimes the floor panel can be made to drop out. Other times either side can be opened. Sometimes the roof will lift off. All usable houses should be cleaned out of previously used nesting materials, pieces of eggshells, excrement, etc. at least once or twice a year. Letting birds use houses that have not been properly maintained may be worse that not providing the nesting box in the first place.

Other features
Good bird houses, those that are made with the considerations of birds above all else, will not have perches near the entrance/exit hole. The perch is totally unnecessary as the birds that can use any natural cavity in trees can also cling to houses without needing the perch. Plus, perches can make it easier for predators to gain access. Mounted birdhouses are preferable to hanging models. Wind gusts can make entry/exit more difficult in hanging models and eggs can roll around inside the house and crack. An escape ladder on the inside of the house, between the hole and the floor, makes it easier for fledging chicks to climb out. A raised platform of metal hardware cloth on the interior floor keeps nesting materials off the floor and provides fewer  opportunities for mites, bacterial growth, and other insect pests to attack newborn chicks. Some houses come equipped with predator guards. These are inexpensive and can be installed separately and inexpensively.

Monitoring
Monitoring the house during breeding season helps insure the survival of the chicks. If you want to observe the nest-building, incubation period, raising of nestlings, and, finally, fledging of the chicks, a view-thru house enables you to see into the house without creating any disturbance to the birds inside. Usually a single panel, a side perhaps, can be left open and a clear Plexiglas window allows viewing and prevents any eggs, babies, or materials from accidently falling out. Monitoring the view-thru house provides hours of stimulating entertainment for adults and children. Additionally, it is a very rewarding and engaging educational experience for children.

What to avoid
Many houses are made and sold for what I call the “novelty” market. That is, those bird houses are made and sold as collectibles. Some folks collect antiques, or art, or coins, etc. And some people collect bird houses. These houses are made for a variety of reasons that appeal to the people who collect them. Their primary concerns may be size, designs, colors, cost, etc. The novelty houses are never built to specification and therefore probably wouldn’t attract birds to use them. They generally tend to be too small for most birds, both in the overall dimensions and in the entrance/exit hole. Most of these types of birdhouses wouldn’t survive the weather very long. These types of houses are found in craft stores, in kit form or preassembled. We’ve had customers who have bought these types of houses only to complain that birds have never used them. They were bought because they had eye appeal to humans and were inexpensive. Many of these houses are imported from China, Malaysia, Thailand, and the Philippines.  Obviously, if you are serious about providing real housing that affords the birds the protections they need, then these bird houses are to be avoided.

Cavity-nesters
In North America there are about 85 species that are cavity-nesters and these are the species you can attract to your backyard bird houses. Cavities are usually excavated by various woodpeckers and flickers, who generally use them first. As the makers of the cavities and the first to actually use them, these birds are referred to as primary cavity-nesters. Other birds that take up abandoned or unused cavities are called secondary cavity-nesters. These cavities will last many years and afford many generations of various species opportunities for roosting and breeding. The woodpeckers and flickers generally create multiple cavities each breeding season, as the male will occupy a single cavity and his mate will occupy a separate cavity with each new brood. The male will further develop another one, two, or three cavities that are left unoccupied, as “dummy” cavities whose purpose is discourage predators.

Importance of saguaros as bird housing

Photo by Doris Evans

Many of the 85 or so species that are known to occupy cavities will also use human-built birdhouses, especially when natural choices are scarce or limited. Bird houses (nesting boxes) can figure prominently in the conservation of many species. Without forests of trees, the Sonoran Desert only offers minimal opportunities for nesting birds. Saguaros are most extensively used in the Sonoran desert for several reasons. They tend to be the tallest plants around and therefore offer excellent observation posts. The height and spines of the cactus discourage many ground dwelling predators and, very importantly, a saguaro cavity offers the inhabitants a naturally cooled living space.  The cavity is surrounded by the moisture of the interior of the cactus and thereby acts as a natural evaporative cooler for the animals inside the cavity. This is critically important to the birds’ survival as the temperature inside the cavity on a very hot summer day will be 20-30 degrees cooler than the outside ambient air temperature. Without the saguaros, fewer birds would be breeding here.

So, if you live out in the desert surrounded by stands of saguaros, it is possible that your wooden bird house may or may not be used by the usual suspects. Certainly it’s possible. But the availability of a cooled apartment may be too enticing. However, if you don’t have many saguaros on your property, then it is much more likely that your bird house will ultimately become occupied. If you live in a suburban or urban area without a plethora of saguaros, then your chances of having occupied bird houses greatly increases. Generally urban and suburban areas offer more and a greater variety of habitats, vegetation, and foods. So naturally there are more varieties of birds closer to human populations than in any single section of the Sonoran Desert. By providing housing for birds, you will be rewarded for years to come as birds claim ownership and breed year after year in the same houses. Resident birds may take up residence year round while migratory birds that breed here can use them certain parts of the year.

Historical record
Cavity-nesters generally tend to be insectivores, but not always. Going back in time, about 100 years ago and earlier, there were a few more species that used natural cavities. But with the advent of “modern” forestry management, we’ve logged more than 99% of our old-growth forests. Even the dead old-growth snags have been harvested. These old-growth snags were the preferred sites for cavity-nesting birds to excavate. The forests provided plenty of insects, shelter, and perfect habitat. Peregrine Falcons and Merlins used these dead snags until around the turn of the last century, when they became no longer available. However, they were more adaptable than the Ivory-billed and Imperial Woodpeckers that excavated the largest tree cavities. Today, as a result of the near total loss of the old-growth forests and their inability to adapt to different habitat, both woodpecker species are assumed to be extinct. It may be possible that a few individuals have survived, but, even if that were the case, they would still be a doomed species for lack of a sufficient breeding population and it will be many years before we experience large stands of old growth again.
Primary cavity-nesters are the birds that actually excavate the cavities. Secondary cavity-nesters are the many other species that will take up residence in abandoned cavities later on. Nearly half of these birds have been documented using human-made bird houses. It’s always interesting, and sometimes surprising, to discover which species have taken up residence in the bird houses we offer.