Bluebird boxes provide nesting sites for native cavity-nesting birds like Eastern Bluebirds, Tree Swallows, and other species. However, non-native and aggressive species can also move into these man-made nest boxes and take them over from the intended residents. Understanding which birds may compete for bluebird boxes can help bird enthusiasts monitor and manage their bluebird trails effectively.
Bluebirds and Tree Swallows
The two most common species found using bluebird boxes in North America are Eastern Bluebirds and Tree Swallows. Bluebirds are year-round residents across much of eastern and central North America. Tree Swallows summer across most of North America and migrate south for winter. Both species readily accept nest boxes.
Bluebirds are typically the earliest spring migrants that return to nesting areas in northern climes. They often stake claim to nest boxes first. However, Tree Swallows arrive not long after. The two species willsometimes compete for ownership of prime nesting sites. Tree Swallows are highly aggressive and will harass and fight off bluebirds to take over a coveted box.
Bluebirds and Tree Swallows will often nest in close proximity, just not in the same box. Proper box placement, monitoring, and management can help reduce competition and allow both species to thrive. Installing pairs of boxes about 100-feet apart, facing opposite directions, and mounted on poles with baffles to deter predators are best practices for bluebird trails.
House Sparrows
The non-native House Sparrow is one of the biggest competitors bluebirds face for nest sites. Introduced from Europe, these birds are found across North America, especially near human habitation. House Sparrows are highly aggressive and will chase off and even kill bluebirds and other native cavity nesters to take over nest boxes.
Sparrows start claiming nesting sites very early in spring. They will fill boxes with nest material but not use them right away. This blocks native species from accessing nest sites when they arrive later in migration. House Sparrows may also peck holes in eggs, kill nestlings, and possibly even adult bluebirds.
Controlling sparrow populations and preventing them from accessing nest boxes is key. Sparrow spookers, entrance size reducers, and vigilant monitoring helps limit this invasive species. Removing sparrow nests as soon as they appear in boxes is also critical.
Other Competitors
Other non-native species like European Starlings may also usurp bluebird boxes. Native woodpeckers, wrens, and chickadees will on occasion take over boxes intended for bluebirds as well. Properly constructed and placed nest boxes with routine monitoring helps minimize competition.
Here are some other species that may compete for bluebird nesting sites:
| Common Name | Scientific Name |
|---|---|
| House Sparrow | Passer domesticus |
| European Starling | Sturnus vulgaris |
| Tree Swallow | Tachycineta bicolor |
| House Wren | Troglodytes aedon |
| Carolina Chickadee | Poecile carolinensis |
| Downy Woodpecker | Dryobates pubescens |
| Northern Flicker | Colaptes auratus |
Preventing Takeovers
Proper bluebird box placement and construction can help deter competitors and allow bluebirds to successfully nest. Here are some tips:
- Use boxes with 1 1/2″ diameter entrance holes, which bluebirds can fit through but exclude starlings
- Place boxes in open, grassy areas away from trees and buildings where competitors congregate
- Mount boxes 5-6 feet high on metal poles with predators guards
- Face boxes away from prevailing winds and direct sunlight
- Clean out old nests in fall to prevent parasites and discourage re-use
- Regularly monitor boxes and remove competitor nests as soon as they appear
Having paired boxes about 100 feet apart allows bluebirds alternative nesting sites if competitors take over one box. Maintaining healthy bluebird habitat with nest sites spaced at proper distances is also key. With vigilance, non-native species can be controlled and bluebird productivity sustained.
Dealing with Takeovers
Even well-maintained bluebird trails may experience occasional nest box takeovers. Here are some options for dealing with them:
House Sparrows
- Remove new sparrow nests as soon as they are spotted
- Install a sparrow spooker over the entrance to discourage sparrows from entering
- Try dipping dried pine needles in hot pepper wax and stuffing the box to deter sparrows
- Attach a nest entrance restrictor allowing bluebirds in but keeping sparrows out
- As a last resort, remove the nest box entirely until the sparrow threat has passed
European Starlings
- Enlarge the nest entrance hole to 2 1/8″ to allow starlings to leave but exclude them from re-entering
- Coat the inside roof of nest box with slippery petroleum jelly to make it difficult for starlings to perch
- Install a starling-resistant crescent opening instead of a circular entrance hole
- Place starling-excluding meshes over vent holes and the roof’s rain guard
- Remove and destroy starling nests and eggs
Tree Swallows
- Monitor closely during spring arrival to allow bluebirds first access
- Hang more than one box in the immediate area to provide alternate sites
- Temporarily tape over entrance holes of boxes being defended by swallows to give bluebirds a chance
- Remove swallow nests early in the season to give bluebirds a head start
Helping Bluebirds
Even when competitors take over some nest boxes, there are ways to assist bluebirds successfully fledge young:
- Keep backup nest boxes clean and available for bluebirds forced out of an occupied site
- During nest building, quickly collect, freeze, and return bluebird eggs laid outside a box to encourage renesting attempts
- Consider providing supplemental feeders with mealworms to help bluebird adults continue renesting
- Monitor bluebird pairs whose first brood fledged and install a nest box guard to give them preference for second broods
With persistence and properly managed bluebird trails, cavities lost to aggressive competitors can often be reclaimed for native species again in future years.
Conclusion
Non-native House Sparrows and European Starlings are the main species likely to overtake bluebird nest boxes. Tree Swallows may also compete for sites against bluebirds. Careful box placement in ideal open habitat, proper entrance hole sizing, and vigilant monitoring helps minimize competition.
When takeovers occur, there are techniques available to exclude or remove problem nests and deter the competitors. Maintaining excess nesting sites helps bluebirds successfully renest if they lose an initial box. With diligent oversight, bluebird enthusiasts can achieve the optimal balance on their trails of bluebirds, Tree Swallows, and other native cavity nesters while limiting impacts of aggressive invasive competitors for the boxes.
