Eastern Bluebird Nest Box Study

Native to the eastern United States, eastern bluebirds are a culturally and ecologically important species that can be found year-round at Mt. Cuba Center. These birds are an important part of the ecosystem, consuming insects and serving as food for larger birds of prey. The striking blue-feathered birds are a symbol of prosperity, hope, and new beginnings in many indigenous and contemporary American cultures.

Now a commonly occurring bird, eastern bluebirds were nearly driven to extinction in the mid-20th century. During that time, habitat loss and competition from non-native, aggressive birds such as the European starling and house sparrow for nesting sites drove dramatic population declines. Despite bleak prospects, citizens across the nation came together to install bluebird nest boxes and protect bluebird habitat, leading to the bird’s recovery in the late 20th century. Inspired by this story of conservation success, MCC installed bluebird nest boxes in the natural lands in the early 2000s.

Since 2012, Mt. Cuba has monitored the nest boxes to collect data to determine their effectiveness in supporting bluebird reproduction. This report examines data collected between 2015 and 2020. Read on for a summary of the report or access the full report at the bottom of the page.

Nest Box Competition

While collecting data from the nest boxes, tree swallows, house wrens, and Carolina chickadees were also observed using the boxes. Though these birds are all native to Delaware, Mt. Cuba is specifically interested in supporting bluebirds. To determine how to reduce competition for the boxes, Mt. Cuba partnered with the University of Delaware (UD) in 2014. Using data previously collected from the nest boxes at Mt. Cuba, Christine Rega-Brodsky, a UD graduate student, performed a statistical analysis to determine what factors influence bluebird nest box selection. The analysis revealed that:

  • The driving force in bluebird nest box selection was distance to shelter (how far away a nest box was from trees/shrubs).
  • Boxes placed between 15 and 30 meters from shelter attracted the most bluebirds and fewest non-target species.

Relocating the Boxes

Before the start of the 2015 breeding season, using results of the UD study, Mt. Cuba moved all nest boxes to locations within 15 and 30 meters from shelter. Following this, Mt. Cuba continued to collect data from the nest boxes to determine if efforts to better attract bluebirds were successful. This report examines data collected from the nest boxes in the five years following the relocation (2015 – 2020).

Post-Relocation Findings

Bluebirds faced most of their competition for nest box space from tree swallows, therefore, this assessment focused on bluebirds and tree swallows. The graph below shows how fledglings* per nest box changed between 2015 and 2020 for each species.

*Fledgling: A young bird that has just left the nest.

Two clear trends emerge from the graph. Following nest box relocation in 2015, bluebird fledglings per nest box increased while tree swallow fledglings per nest box decreased. This does not necessarily mean that the local bluebird population is increasing while the tree swallow population is decreasing. Rather, it provides evidence that Mt. Cuba successfully placed boxes in locations that are more attractive to bluebirds than tree swallows and other species.

Support Bluebirds at Home

If you have a yard with open space, you can attract eastern bluebirds by installing a nest box in a good location. Box placement is very important and will determine what kind of birds you bring to your yard. Based on the UD study and our data analysis, Mt. Cuba recommends the following to support bluebirds:

  • Install nest boxes in a spot that is open, sunny, and preferably surrounded by low-cut grass.
  • Place boxes 15 to 30 meters away from tree-line, shrubby border, or other tree-like habitat.

Other recommendations:

  • Install nest boxes at least 50 feet from home and any bird feeders
  • Face the box toward open space so birds can fly directly into it
  • Place nest box about 5 feet off of the ground on a metal or wooden pole
  • If using a wooden pole, install a metal baffle below the nest box to protect young birds from climbing predators (e.g. snakes, squirrels, raccoons, cats)

For more information on eastern bluebirds and nest boxes, check out the sites below.
Nest Watch: Tips for nest box placement
Audubon: Bluebird basics and directions for building a nest box
Michigan Bluebirds: Directions for building a nest box, nest box placement, maintenance, and monitoring

Curious about what bluebirds eat? Check out this video shot from the top of bluebird nest boxes to find out.

Click through the pages below to read the full report on the Eastern Bluebird Nest Box Study.