2021 Was Not a Good Year for Bluebirds

ConservationEncounters with Nature
Written By Kevin Smith

By Bob Parker, Boulder County Audubon Bluebird Coordinator

2021 was a disappointing year for Mountain and Western Bluebirds in Colorado. The data from our Boulder County Open Space and Boulder County Audubon bluebird monitoring combined with the data from the entire state of Colorado bluebird box surveys shows fewer bluebirds fledged in 2021 than in 2020. The decline wasn’t due to the number of birds fledged per nest but instead the total number of nests. There were significantly fewer overall nesting attempts in 2021 than in 2020.  Something happened to reduce the number of birds returning to nest in 2021 and a likely culprit is the 2020 drought which was quite severe over a significant portion of their migration route.

Bluebird Nesting Cycle Photo Gallery

These five photos show typical progression through a Mountain Bluebird nesting cycle. Mountain Bluebirds have an incubation period of 18-21 days and then remain in the nest for 18-25 days before fledging.

Boulder County Bluebird Box Route Summary

Boulder County Audubon has operated ten bluebird nest box routes on Boulder County Open Space properties for over 15 years. There are four routes on Walker Ranch, two routes on Betasso Preserve, two routes on Heil Ranch, one on Bald Mountain, and one on Minnick (along the Peak to Peak Highway north of Nederland). Both routes on Heil Ranch were destroyed by the Cal Wood fire in October 2020 and have not yet been replaced.  Hopefully, the county open space will give us permission to replace the boxes for the 2022 season, as bluebirds are one of the first species to reenter burned forests. To learn more about bluebirds and other early species in burned areas, All About Birds has a great article. 

Most of the boxes on one of the routes at Betasso and at Minnick were moved in 2021 to better locations.  As the trees grow, the habitat changes and some of the boxes cease being attractive to the bluebirds. Boxes occasionally need to be moved on all of the routes to ensure we continue to monitor in habitat appropriate for bluebirds.  Additionally, every year a few boxes are destroyed by bears, chewed by squirrels or raccoons, and just wear out.  These boxes may be replaced or moved. 

2021 had the lowest nest box occupancy at Walker Ranch in the last six years (Table 1). The Walker Ranch Yellow route circles the open field around the main ranch house east of the Walker Ranch Loop Trailhead and consists of 11-12 boxes in ideal bluebird habitat. From 2016 through 2020, we had only one unoccupied box across all five years. In 2021, four of the boxes were empty. Tree and Violet-green Swallows and House Wrens also nest in these boxes. The number of swallows and House Wrens occupying the boxes in 2021 was consistent with past years. So empty boxes seem to be entirely due to fewer bluebirds nesting in 2021.  Hopefully we will see more bluebirds in 2022, as we did in 2019 after a down year in 2018!

Table 1. Walker Ranch yellow route nest box occupancy in 2021. If a box had more than one species occupy it per season only the first nest is counted.

Total Boxes Mountain Bluebirds Western Bluebirds % with Bluebirds Tree & VG Swallows House Wren Empty % Empty
2016 12 10 0 83.3% 2 0 0 0.0%
2017 11 9 0 81.8% 3 0 0 0.0%
2018 11 5 1 54.5% 3 1 1 9.1%
2019 12 8 1 75.0% 2 1 0 0.0%
2020 12 7 1 66.7% 3 1 0 0.0%
2021 12 4 0 33.3% 2 2 4 33.3%

Colorado Bluebird Box Monitoring Summary

Using data provided by Colorado state bluebird coordinator Kevin E. Corwin for 2020 and 2021 from the state’s 1500 or so bluebird boxes (Tables 3 and 4 at the end of the article), I find that the patterns seen in Boulder county hold true across the state (Table 2).

Table 2. Percent increase in nesting-related metrics between 2020 and 2021 for all species recorded by the Colorado Bluebird Project (negative numbers represent a decrease in 2021). Data for Juniper Titmouse, Barn Swallow, and bluebird sp. are excluded from this table as they had zero nesting attempts in either 2020 or 2021.

Species

Nest Attempts

Total Eggs

Unhatched Eggs

Nestlings

Fledglings

Mountain Bluebird

-35.8%

-35.3%

-20.7%

-38.6%

-39.1%

Tree Swallow

-14.9%

-13.2%

-24.4%

-8.4%

-18.2%

Western Bluebird

-47.4%

-47.5%

-38.6%

-52.2%

-50.4%

Violet-green Swallow

6.5%

3.6%

5.7%

-0.5%

4.0%

House Wren

-9.6%

-7.2%

260.0%

3.9%

0.0%

Mountain Chickadee

35.0%

55.1%

66.7%

53.0%

64.6%

White-breasted Nuthatch

45.5%

45.5%

100.0%

32.7%

41.9%

Pygmy Nuthatch

16.7%

42.0%

-63.2%

78.0%

104.7%

Tree/Violet-green Swallow

300.0%

278.6%

NA

225.0%

141.7%

Ash-throated Flycatcher

133.3%

163.6%

500.0%

110.0%

112.5%

swallow sp.

-57.1%

-30.0%

-33.3%

-36.4%

-45.5%

Black-capped Chickadee

133.3%

125.0%

200.0%

52.9%

76.9%

Total Across All Species

-25.2%

-23.9%

-21.9%

-24.6%

-27.2%

Swallows and bluebirds, the most common residents in our Boulder County Audubon boxes, are also the most common across the state.  Mountain Bluebirds had 36% fewer nesting attempts and 39% fewer fledglings in 2021 relative to 2020.  Western Bluebirds suffered a 47% decline in nesting attempts and 50% fewer fledglings in 2021 relative to 2020.  Tree Swallows fared a little better with only a 15% decline in nesting attempts and an 18% decline in total fledglings.  Chickadees and nuthatches showed an opposite pattern with both increasing their nesting attempts and fledging numbers, however, these species have quite low occupancy of nest boxes and so a small change in the number of nests means a dramatic percentage increase. House Wren nesting attempts and total fledglings were similar in the two years.  

You will also notice a total of 25% fewer nesting attempts overall in 2021 relative to 2020.  This was due mostly to empty boxes, and not to a reduction in the number of boxes available. We hope that more boxes will be used in 2022 and that the drought conditions will be vastly improved compared with past years.

Conclusions

Western and Mountain Bluebird nesting attempts suffered declines of 47% and 36%, respectively, in Colorado in 2021 compared to 2020.  Average eggs per attempt and nestlings per attempt were very similar in 2021 and 2020, so the reduction in birds was due to fewer nests attempted in 2021.  I believe that drought conditions along the migration route and/or in their wintering grounds were responsible for the reduction in nesting attempts in 2021.  It is hoped that more birds will return to nest in 2022 and numbers will begin to recover.

Table 3. Summary of all activity in Colorado Bluebird Project nest boxes for 2020. Data compiled by Kevin E. Corwin.

Species Nesting attempts Eggs Unhatched eggs Nestlings Fledglings
Mountain Bluebird 531 2583 382 2101 1921
Tree Swallow 483 2401 349 1862 1564
Western Bluebird 302 1499 228 1227 1080
Violet-green Swallow 62 251 35 195 174
House Wren 52 291 5 229 211
Mountain Chickadee 20 98 6 83 82
White-breasted Nuthatch 11 55 2 49 43
Pygmy Nuthatch 12 69 19 50 43
Tree/Violet-green Swallow 3 14 0 12 12
Ash-throated Flycatcher 3 11 1 10 8
swallow sp. 7 20 6 11 11
Black-capped Chickadee 3 20 3 17 13
Juniper Titmouse 0 0 0 0 0
bluebird sp. 0 0 0 0 0
Barn Swallow 1 4 1 3 3
Total Observed 1490 7316 1037 5849 5165

Table 4. Summary of all activity in Colorado Bluebird Project nest boxes for 2021. Data compiled by Kevin E. Corwin.

Species Nesting attempts Eggs Unhatched eggs Nestlings Fledglings
Mountain Bluebird 341 1671 303 1289 1169
Tree Swallow 411 2084 264 1705 1280
Western Bluebird 159 787 140 586 536
Violet-green Swallow 66 260 37 194 181
House Wren 47 270 18 238 211
Mountain Chickadee 27 152 10 127 135
White-breasted Nuthatch 16 80 4 65 61
Pygmy Nuthatch 14 98 7 89 88
Tree/Violet-green Swallow 12 53 8 39 29
Ash-throated Flycatcher 7 29 6 21 17
swallow sp. 3 14 4 7 6
Black-capped Chickadee 7 45 9 26 23
Juniper Titmouse 1 5 0 5 5
bluebird sp. 1 8 0 8 8
Barn Swallow 0 0 0 0 0
Total Observed 1114 5564 810 4412 3762
Kevin Smith