by Rachel Hutchison
This is the second year of expanded nestbox monitoring in the four Jeffco Open Space Parks of Evergreen and Conifer. The goal of the Evergreen Audubon nestbox monitoring program is to collect data that can be used to determine the causes of success or failure of our bird populations. The results of the last two years have been very similar in the totals of young birds that have fledged. This year’s nesting season stretched well into August, which is a bit longer than usual. The exact reason for this must be guessed. Many boxes saw a second nesting attempt, perhaps because of the early start of warm weather and a consistent supply of insects. Second nests had about the same success rate as first-of-the-season nests (50% to 51%).
Challenges faced by our common box-nesting birds are numerous and, in some instances, difficult for us bird lovers to mitigate. Swallows, bluebirds, and wrens must migrate twice to begin a new nesting season in the Bear Creek Watershed area: first a fall southward journey and then a spring northward trip. The fact that so many of them manage to return each year is great news. Birds must also overcome difficulties in the parks including people, bears, cats, other birds, rodents, wildfire, disagreeable weather conditions, disease, insecticides, and pesticides. Man-made changes to habitat such as residential encroachment, invasive noxious plants, cutting native grasses and trees, and filling in wetlands add to a decrease in acceptable acreage. Another factor is global warming which can cause a disconnect between when insects are plentiful and when birds need nutrition to gain strength after a migration to be healthy enough to lay eggs, and again when the young need huge amounts of insects to mature to fledge and migrate.
Twenty-one awesome volunteer monitors from Evergreen Audubon diligently checked 157 nestboxes on a weekly basis, checking each of the nestboxes in their respective areas an average of 14 times each during the nesting season. This came to a monumental number of 2,090 individual box visits. Weekly visits are necessary to correctly record the number of eggs laid, number that hatched, and the number of baby birds that fledged. Recording predation information is also part of monitoring. This year six boxes were torn down by bears compared to twelve last year. Moving one group of 14 boxes to the other side of the Open Space Park may have accounted for this decrease. House wrens accounted for a significant number of predations on their fellow wren, bluebird, and swallow nests.
Of 470 hatched eggs, 407 survived to leave the nests. While these overall numbers compare similarly to 2024 results, there were some significant differences in species numbers. This year no Pigmy or White-breasted nuthatches had successful nests, whereas last year there were three such nests. This year also saw an increase in nests and number of eggs laid by Western Bluebirds from last year, yet still greatly reduced from the results seen nine years ago. Mountain Bluebirds and Mountain Chickadees maintained approximately the same numbers as last year. House wrens were down from last year – maybe the result of an overabundance due to last year’s success. House Wrens are highly territorial and will sabotage their fellow wren nests. This was recorded by our monitors.
Since we do have data available to compare results between 2025 and 2017 for Elk Meadow and Alderfer/Three Sisters let’s look at some numbers to get an idea of how the bird production in those two parks has differed in the last eight years. Elk Meadow nestboxes produced 86 Mountain Bluebirds in 2025 compared to 25 in 2017. Alderfer/Three Sisters produced 13 in 2025 compared to 8 in 2017. This is a most significant improvement for Mountain Bluebird numbers in these two parks. Our other highest number species are Tree Swallows. Elk Meadow numbers increased from 46 to 51 in those years. Alderfer/Three Sisters numbers decreased from 8 to 4.
Points to remember when comparing years is that both parks recently had major fire mitigation activity causing changes to the habitat and disruption due to heavy equipment activity. Parts of both parks were closed to people for a significant part of the nesting season.
Some of these changes may be good for birds. Monitoring over the coming years may give data to further help our wildlife.
| Table 1. Breeding Success by Open Space Park | |||||||
| Bluebirds | Swallow | Mountain | House | ||||
| Mountain | Western | Tree | Violet-Green | Chickadee | Wren | ||
| Alderfer/Three Sisters | Nests with eggs in 2025 | 7 | 3 | 4 | 1 | 3 | 0 |
| Successful nests in 2025 | 3 | 2 | 3 | 1 | 2 | 0 | |
| Average clutch size in 2025 | 5.1 | 5.3 | 4.5 | 4.0 | 5.3 | — | |
| % hatched eggs laid in 2025 | 52.8% | 56.3% | 83.3% | 75.0% | 81.3% | — | |
| % fledged of eggs laid in 2025 | 36.1% | 56.3% | 44.4% | 75.0% | 81.3% | — | |
| Chicks fledged/ pair in 2025 | 1.9 | 3.0 | 2.0 | 3.0 | 4.3 | — | |
| Young produced in 2025 | 13 | 9 | 8 | 3 | 13 | 0 | |
| Elk Meadow | Nests with eggs in 2025 | 21 | 0 | 17 | 0 | 0 | 3 |
| Successful nests in 2025 | 19 | 0 | 15 | 0 | 0 | 3 | |
| Average clutch size in 2025 | 5.3 | — | 4.5 | — | — | 7.7 | |
| % hatched eggs laid in 2025 | 82.9% | — | 85.5% | — | — | 100.0% | |
| % fledged of eggs laid in 2025 | 77.5% | — | 67.1% | — | — | 100.0% | |
| Chicks fledged/ pair in 2025 | 4.1 | — | 3.0 | — | — | 7.7 | |
| Young produced in 2025 | 86 | 0 | 51 | 0 | 0 | 23 | |
| Flying J Ranch | Nests with eggs in 2025 | 9 | 2 | 8 | 0 | 6 | 8 |
| Successful nests in 2025 | 6 | 2 | 3 | 0 | 4 | 6 | |
| Average clutch size in 2025 | 4.0 | 5.5 | 3.9 | — | 6.2 | 6.6 | |
| % hatched eggs laid in 2025 | 77.8% | 100.0% | 71.0% | — | 83.8% | 100.0% | |
| % fledged of eggs laid in 2025 | 77.8% | 100.0% | 38.7% | — | 59.5% | 79.2% | |
| Chicks fledged/ pair in 2025 | 3.1 | 5.5 | 1.5 | — | 3.7 | 5.3 | |
| Young produced in 2025 | 28 | 11 | 12 | 0 | 22 | 42 | |
| Meyer Ranch | Nests with eggs in 2025 | 9 | 2 | 3 | 2 | 2 | 2 |
| Successful nests in 2025 | 9 | 2 | 2 | 2 | 2 | 2 | |
| Average clutch size in 2025 | 5.1 | 5.0 | 4.3 | 4.5 | 2.5 | 7.0 | |
| % hatched eggs laid in 2025 | 95.7% | 90.0% | 38.5% | 100.0% | 100.0% | 100.0% | |
| % fledged of eggs laid in 2025 | 95.7% | 90.0% | 38.5% | 100.0% | 100.0% | 100.0% | |
| Chicks fledged/ pair in 2025 | 4.9 | 4.5 | 1.7 | 4.5 | 2.5 | 7.0 | |
| Young produced in 2025 | 44 | 9 | 5 | 9 | 5 | 14 | |
| Table 2. Breeding Success Totals | |||||||
| Bluebirds | Swallow | Mountain | House | ||||
| Mountain | Western | Tree | Violet-Green | Chickadee | Wren | ||
| Totals | Nests with eggs in 2025 | 46 | 7 | 32 | 3 | 11 | 13 |
| Successful nests in 2025 | 37 | 6 | 23 | 3 | 8 | 11 | |
| Average clutch size in 2025 | 5.0 | 5.3 | 4.3 | 4.3 | 5.3 | 6.9 | |
| % hatched eggs laid in 2025 | 79.9% | 78.4% | 77.5% | 92.3% | 84.5% | 100.0% | |
| % fledged of eggs laid in 2025 | 74.7% | 78.4% | 55.1% | 92.3% | 69.0% | 87.8% | |
| Chicks fledged/ pair in 2025 | 3.7 | 4.1 | 2.4 | 4.0 | 3.6 | 6.1 | |
| Young produced in 2025 | 171 | 29 | 76 | 12 | 40 | 79 | |