September Nature Almanac: Songbirds of Eastern Deciduous Forests Finding New Nesting Sites in Boulder County
By Stephen Jones with Ruth Carol Cushman
September 2025
When Curt and Karen Brown saw a Rose-breasted Grosbeak at their South Boulder feeder in late August, they weren’t terribly surprised until they noticed the scruffy condition of the bird. With its ruffled feathers and incomplete rose patch, it looked for all the world like a recently fledged juvenile male.
Juvenile Rose-breasted Grosbeak. Photo by Curt Brown.
While rose-breasted grosbeaks have become increasingly common in Boulder County during the last couple of decades, the vast majority of observations are of migratory birds seen or heard in May and June. In fact, over the years there have been only a couple of observations of nesting pairs in Boulder County and fewer than a dozen throughout Colorado.
The breeding range of these denizens of eastern deciduous forests barely overlaps that of our locally common Black-headed Grosbeaks. The Rose-breasted Grosbeaks commonly nest from the western Great Plains eastward to the Atlantic Seaboard; and the Black-headed Grosbeaks breed from the Great Plains westward to British Columbia, California, the Baja Peninsula; and south into Central Mexico. Both species nest in deciduous trees, including cottonwoods, and that may partially explain why so many hybrid pairs are seen in areas of western Nebraska and eastern Colorado.
Orchard Oriole. Photo by Gerhard Assenmacher.
Other species of eastern deciduous forests that have recently taken up residency in Colorado include Orchard Oriole, Northern Cardinal, and Least Flycatcher. Orchard Orioles have become increasingly more common in Boulder County over the past couple of decades, with pairs nesting at Sawhill Ponds, White Rocks Natural Area, and in Longmont. In some riverine habitats of the western plains, especially those with dense shrubbery and widely scattered trees, they now outnumber both Bullock’s and Baltimore Orioles.
Northern Cardinals have nested in dense riparian woodlands along Colorado’s eastern border for several decades, and one or two individuals have been seen almost annually in North Boulder for 20 or more years. We still haven’t confirmed nesting of cardinals in Boulder County, but nesting has been confirmed in Denver. Some observers believe that the Colorado Front Range does not have enough of the dense, prickly thickets that cardinals prefer for nesting; but shrub vegetation in our foothills canyons, including the canyon immediately west of the area in North Boulder where cardinals are most frequently seen, has grown much more dense over the years with removal of grazing cattle and control of local brush fires.
Northern Cardinal. Photo by Gerhard Assenmacher.
Least flycatchers, first reported breeding in Colorado during the 1987-94 Colorado Breeding Bird Atlas project, have been confirmed nesting recently in eastern Boulder County, south of Denver, and on the Colorado Western Slope.
What all these species have in common is an affinity for deciduous woodlands, and as deciduous trees continue to spread along our streams and across our remaining grasslands, we can expect to see more of them in eastern Colorado.
As for the apparent juvenile male on Curt and Karen’s bird feeder, we looked online at dozens of images of recently fledged Rose-breasted Grosbeaks, and nearly all of them showed no red at all on their breasts. So perhaps this bird fledged very early in the summer and is on his way south for winter; or maybe he’s a one-year-old who simply got caught out in one of our recent downpours.
Other September Events:
Evening Grosbeak flocks zigzag through the conifers seeking out wild fruits, budworms, and seed crops, stopping occasionally at mountain bird feeders. Their numbers have plummeted in Boulder County recently, and no one seems to know why.
Pine Grosbeaks sunbathe on the ground in Engelmann-spruce/subalpine fir forests, just below tree line. In winter these relatively tame seed-eaters gravitate toward salted and sanded roads, where many are killed by automobiles. The striking red males and soft yellow-gray females remain together year-round, sharing food and cooperating in the rearing of their young.
Hawks and falcons ride September thermals south as they migrate toward wintering areas from the central Rockies to Argentina.
Hundreds of thousands of ladybird beetles (“ladybugs”) gather on granite outcrops of Green Mountain and Bear Peak, eventually crawling into cracks to hibernate—assuming that hungry bears don’t find them first.
Ladybug aggregation on Green Mountain. Photo by Stephen Jones.
Nature Almanac is a monthly series by Stephen R. Jones and Ruth Carol Cushman, along with other guest contributors. Ruth Carol Cushman and Stephen Jones are authors of A Field Guide to The North American Prairie (Peterson Field Guides) and Wild Boulder County: A Seasonal Guide to the Natural World.