Colorado Bluebird Project 2020 Nesting Success

Colorado Bluebird Project 2020 Nesting Success

2020 NestWatch Statistics for Colorado’s Cavity Nesters
Kevin Corwin, Colorado Bluebird Project Chairman

Hello Fellow Bluebirders,

The chart below lays out the nesting success of the cavity-nesting birds (mostly nest box users) that were reported to Cornell’s NestWatch program in 2020. 2020 was a pretty good year for our monitors across the state.

It’s important to note that our reports show just the tip of the nesting productivity in the state, far more cavity-nesters raise their broods out of sight and awareness of any humans. For example, the first Colorado Breeding Bird Atlas, published in 1998, culminated in an intensive seven-year state-wide project to census all breeding bird species in Colorado. Their minimum estimate for adult Mountain Bluebird breeding pairs in Colorado was 114,527. We counted 531 nest attempts for Mountain Bluebirds, which accounts for no more than—and probably less than—that number of breeding pairs.

But this does not diminish the importance of what we do. Scientists and researchers who use the NestWatch database can extrapolate from the data we provide them to the broader populations of cavity-nesters. And each of us, as we monitor our boxes year after year, develop our awareness of the variables that affect the birds nesting on our trails and become better at fixing what we can change and accepting what we can’t change (like the weather). Every one of you, through your personal observations and actions, and your reporting to NestWatch, is an important part of the network of people in North America who are working to improve bird populations across the continent.

I thank you, the scientsts thank you, and the birds thank you.

Bluebird Monitoring in Castle Rock
Barbara Spagnuolo

The Town of Castle Rock (Douglas County) has finally wrapped up the 2020 season of our 190 bluebird nest boxes. The monitoring data has been collected, reviewed, analyzed and completely transferred to NestWatch. I highly encourage folks to visit our project webpage at to view a summary presentation that features 2020 highlights, graphs of our monitoring data as well as quotes and other information.

Here is a summary of the highlights of the 2020 season in Castle Rock:

  • 190 next boxes were monitored by 105 volunteers contributing over 1000 hours
  • Volunteers had many positive encounters with the public during the monitoring that kicked off just as the COVID-19 lockdown began, providing a great opportunity to talk about this project
  • 6 different native bird species used our nest boxes
  • House Wrens nested in our boxes for the second year in a row, after multiple dummy wren nests were found
  • A Mountain Chickadee pair nested in the same box this year as last year
  • Although we didn’t have a late May snowstorm like 2019 to contribute to mass nest failures, a late April freeze did affect several nests with eggs
  • 92% of the nest boxes were occupied this season
  • We had our latest recorded fledge date of August 29
  • 2020 fledge numbers by species (total 789 fledglings):
    • Mountain Bluebird: 365
    • Tree Swallow: 265
    • Western Bluebird: 144
    • House Wren: 9
    • Mountain Chickadee: 4
    • Violet-green Swallow: 2

May all your blues be birds!

Photo credits (from top to bottom): Mountain bluebird inside nest box by Jill Holden; Mountain bluebird at nest box by Pat Brodbent; Pair of Mountain bluebirds by Bob Andrews; Pair of Western bluebirds on box by Gina Gerken; Male Western bluebird at box by Gina Gerken; Western bluebird on no parking sign by Dick Vogel; Tree swallow in box 47 by Cynthia Madsen; Tree swallow fledglings by Hugh Kingery; Tree Swallow posed with flared wings by Dick Vogel; Tree & Violet-green swallows by Glenn Walbek; Mountain bluebird fledgling by Tim Deger