House sparrows (Passer domesticus) and bluebirds (genus Sialia) are two common backyard bird species in North America that often utilize similar nesting habitats. This has raised questions among bird enthusiasts and researchers about whether these species compete with each other, and if so, how this competition affects their populations.
What are house sparrows and bluebirds?
House sparrows are small birds native to Europe and Asia that have been introduced around the world by humans. They are very adaptable generalists that can thrive around human habitation. Bluebirds are native North American species that nest in cavities like old woodpecker holes and birdhouses. There are three main species: Eastern Bluebird, Western Bluebird, and Mountain Bluebird.
Do they compete for nest sites?
House sparrows and bluebirds both rely on cavities for nesting, so there is significant overlap in their nesting requirements. This suggests they likely compete for limited nesting sites at least in some areas where their ranges overlap. However, the outcome of this competition varies by location and the populations of each species present.
Evidence for competition
Several lines of evidence indicate that house sparrows do competitively exclude native bluebirds from nest sites in many locations:
Similar nesting biology
As cavity nesters, house sparrows and bluebirds require the same key limited resource – holes and crevices to nest in. This automatically sets up a scenario for potential competition.
| Species | Nesting Habits |
|---|---|
| House sparrow | Cavity nester, utilizes birdhouses, nest boxes, and natural cavities |
| Eastern Bluebird | Cavity nester, prefers nest boxes and natural cavities |
House sparrows evict bluebirds from nests
House sparrows are highly aggressive and have been documented attacking and evicting bluebirds from nesting cavities. They destroy eggs, kill nestlings, and even kill adult bluebirds sitting on nests. This frees up the nest site for the house sparrows.
House sparrow presence reduces bluebird numbers
Studies comparing areas with and without house sparrows consistently show lower numbers of bluebirds in places where house sparrows are established. For example, one study in Missouri found areas with house sparrows had only half as many bluebird nests.
House sparrow removal helps bluebirds
Multiple studies have shown that removing house sparrows leads to increased bluebird numbers and nesting success. One nest box program in Oklahoma saw Eastern Bluebird numbers increase from 5 to over 100 pairs after removing house sparrows.
Exceptions and limitations
However, there are some important caveats to the competition between house sparrows and bluebirds:
Habitat preferences differ
While there is overlap in nest site requirements, bluebirds strongly prefer more open, grassy areas, while house sparrows thrive in urban and suburban developed areas. This separation in habitat preferences reduces competition in many natural areas.
Competition outcome depends on densities
Studies show competition is most intense where there are high densities of both species. When house sparrow numbers are low, there may be little effect on bluebird populations.
Other threats also affect bluebirds
Bluebird numbers are also impacted by factors like nest predation, nest site availability, and winter habitat quality. So competition with house sparrows is not the sole driver of bluebird population trends.
Bluebirds sometimes prevail in competitions
Bluebirds will sometimes successfully defend nest sites against house sparrows. Adult bluebirds are often able to repel house sparrows attempting to invade an occupied nesting cavity.
| Species | Clutch Size | Broods per Season |
|---|---|---|
| House Sparrow | 4-6 eggs | 2-5 |
| Eastern Bluebird | 3-7 eggs | 1-3 |
Management implications
The evidence for competition where these species co-occur suggests some management strategies for supporting bluebird populations:
Monitor nest boxes to remove house sparrow nests
Active monitoring and removal of house sparrow nests from nest boxes can open up more potential nesting opportunities for bluebirds.
Set out nest boxes in more open areas
Locating nest boxes in open fields away from dense vegetation and developed areas gives bluebirds an advantage over house sparrows.
Limit food and nesting sites near houses
Restricting access to food sources and nesting cavities in and on human structures can help limit house sparrow densities in areas where bluebirds are nesting.
Educate the public on removing invasive house sparrows
Outreach to inform the public about the negative impacts of house sparrows and how to remove their nests can get more citizens involved in conservation efforts for native bluebirds.
Conclusion
The preponderance of evidence indicates that competition with invasive house sparrows negatively impacts native bluebird populations in many parts of North America where these species co-occur. However, the outcome of this competition is complex and situation-dependent, modulated by habitat differences, population densities, and behavioral interactions between the two species. Thoughtful management and conservation strategies can potentially reduce competition pressure and support native bluebird populations. Further research to clarify the subtleties of competition between these species across various regions would help refine our understanding of this conservation issue.
