A birdhouse, or nest box, serves two primary functions: a secure cavity for spring and summer nesting, or a temporary shelter, called a roost box, for cold weather protection. For the majority of its purpose, the box should not be filled with materials by the homeowner. Understanding the specific needs for each season is the first step in providing a beneficial habitat for cavity-nesting birds.
The Rule of Emptiness: Nesting Material Guidance
For the nesting season, the interior of the birdhouse should be completely empty and dry. Cavity-nesting birds, such as bluebirds, chickadees, and wrens, have an innate drive to select and arrange their own nesting materials. They use local resources like pine needles, grasses, and small twigs to construct a nest suited to their species’ needs. Pre-filling the box with foreign materials can deter birds from using the house, as it interferes with their natural selection process.
Introducing materials can compromise the integrity of the nest box environment. Foreign items may introduce parasites, mold spores, or chemicals that are harmful to developing eggs and nestlings. Materials placed by humans might also compress over time, reducing air space or blocking drainage holes in the floor of the box. The only beneficial feature inside a nesting box is a roughened surface, such as grooves or mesh, below the entrance hole to help fledglings climb out.
Preparing for Winter: Roosting Inserts
The exception to the rule of emptiness occurs when preparing a birdhouse for use as a temporary shelter during the colder months. A roosting box provides a refuge from harsh weather and is not intended for nesting or raising young. To maximize warmth, modifications are typically made, such as covering ventilation holes near the roof to trap rising heat. Small birds often huddle together on cold nights to conserve energy.
A temporary layer of non-toxic, natural material can be added to the floor of the box for insulation. A few handfuls of clean, dried grasses, pine shavings, or straw provide a beneficial barrier against the cold floor. Fine sawdust should be avoided as it can retain moisture and lead to mold growth. After the danger of hard freezes has passed, this temporary roosting material must be removed to prepare the box for the nesting season.
Internal Safety Hazards to Avoid
Many common household materials pose serious safety hazards inside a birdhouse or as nesting material. Dryer lint, often mistakenly offered for its softness, is dangerous because it lacks structural integrity and falls apart when wet. When damp, it can become sticky, coating feathers and trapping moisture against the birds’ skin, which leads to hypothermia. The lint also carries chemical residues from detergents and fabric softeners that are toxic to birds.
Long strands of material, including human hair, pet fur, string, or yarn, should never be placed inside a box. These materials present a severe entanglement risk, which can lead to injury, loss of circulation, or death for young nestlings. Similarly, wood used in the construction or repair of the house must be untreated, as chemicals in pressure-treated wood are toxic and can be absorbed by the birds. Avoid synthetic materials or plastics, which do not break down naturally and can create a suffocating or abrasive environment.
Sanitation and Seasonal Cleaning
Maintaining a clean birdhouse is necessary to ensure the safety of future occupants. Old nesting material, once a brood has fledged, can harbor parasites, mites, and bacteria that pose a serious threat to the next family. The ideal time for cleaning is after the last brood has left, typically in late summer or early fall. This timing also prepares the box for potential winter roosting.
To clean the box, first remove all old nesting material and accumulated debris, wearing gloves and a face mask to avoid inhaling dust or spores. For sanitization, scrub the interior with a weak solution of one part unscented bleach to nine parts water, or a solution of 20% white vinegar and 80% water. After scrubbing, the house must be rinsed thoroughly with clean water to remove all traces of the cleaning agent. The final step is to allow the birdhouse to air-dry completely in the sunlight for several hours before re-mounting, ensuring no residual moisture remains to promote mold or mildew growth.