Bluebirds are secondary cavity nesters that rely on existing holes or artificial nest boxes for shelter. They do not typically reuse the physical nest structure built from grasses and pine needles for subsequent broods. However, they frequently return to and reuse the same nesting cavity or box multiple times within a single nesting season.
Distinguishing Nest Reuse from Cavity Reuse
The tendency of bluebirds to return to a successful breeding location is known as high site fidelity. This means they will often choose the exact same nest box or tree cavity for their second or third brood of the year. Upon returning, the female will construct a brand new nest inside the cavity, even if the old material remains.
The physical nest is the cup-shaped structure made of fine grasses, pine needles, or other materials that holds the eggs and young. The cavity is the surrounding enclosure, such as a wooden nest box or a hole in a tree. Bluebirds will either build the new nest directly on top of the old one, or construct a completely fresh nest if the old material has been removed.
The Biological Necessity of New Nests
The primary reason bluebirds instinctively build a new nest is the accumulation of waste and ectoparasites. A used nest becomes contaminated with fecal matter and residual food, creating an unhygienic environment that can harm the next clutch of nestlings.
A greater biological concern is the buildup of blood-feeding parasites, such as mites, ticks, and especially blowfly larvae. Female blowflies lay their eggs in the nest material, and the resulting larvae emerge to feed on the nestlings’ blood, primarily at night. Such heavy infestations can weaken or even kill young birds.
By abandoning the old nest structure, the parents reduce the parasite load for the next brood. The incidence of blowfly infestation often increases with each successive nesting attempt as the summer progresses, making the instinct to build a fresh, clean nest a strong evolutionary advantage for offspring survival.
Managing Nest Boxes Between Broods
Landowners can improve nesting success by removing old nesting material between clutches. This procedure should only be performed once the previous brood has definitively fledged and the box is completely empty. All native bird nests are protected under the Migratory Bird Treaty Act (MBTA), making it illegal to disturb an active nest containing eggs or young.
Once the box is confirmed to be vacant, the old nest can be safely removed and discarded at least 15 feet away from the site to avoid attracting predators. The cavity should then be scraped clean of any remaining debris, using a tool like a putty knife or a small brush. A mild solution, such as a 1:10 bleach-to-water ratio, can be used to sanitize the box, though it must be allowed to thoroughly air-dry before the bluebirds return to build their new nest.