Species, Date, Time and Location Information
10/10/2019 4:30:00 PM
10/10/2019 4:33:00 PM
Duration (total time in view):
South Platte Park
Multiple Birds - Mixed
Multiple Birds - Mixed
Multiple Birds - Mixed
Open, water, edge, suburban. Series of reservoirs surrounded by cottonwoods and brush.
Decent, overcast sky kept glare and backlighting to a minimum in the diminishing afternoon sunlight.
Description of the Bird
Earlier in my walk around Eaglewatch Lake, I observed a large flock (estimated to be 40 individuals) of Mountain Bluebirds fly low overhead, heading east, and land across the lake in some tall cottonwoods. About two hours later, on my way out of the park, I approached the area where I saw the Mountain Bluebirds land earlier and noticed a few bluebirds there. As I scanned, I came across five individuals that looked different than the nearby Mountain Bluebirds (plumage and structure-wise). These individuals were bluebirds of the same size, however their bills appeared more stout. The two adult males were the most noticeably different, as the blue on the head, wings, rump, and tail appeared a deeper ocean blue, rather than the sky blue of Mountain Bluebirds. Additionally the rufous on the breast was bold, and noticibly extended around the shoulder and onto the scapulars and mantle. The three females/immatures gave me the most challenge, as I had seen several warm-colored female Mountain Bluebirds early in the day. These individuals were perched next to the obvious male Western Bluebirds and were structurally similar, including the stoutness of the bill. The females had the same gray bellies as the Mountain Bluebird females, but the rufous appeared brighter and extend into the scapulars. I did not take note of primary projection or tail length, nor throat color. This sighting seems to corroborate with a pattern of bluebird sightings in Jefferson and Douglas counties following the dramatic arrival of a cold-front overnight.
Similar Species Discussion
These five individuals were immediately recognized as bluebird species based on: their small and compact thrush-like appearance; shades of blue present on the head, wings, and tail; and their gregarious gathering behavior in the tops of bare trees.
All three bluebird species are possible at South Platte Park, through Mountain Bluebird is the most expected species. The individuals in question were apart of a flock of MOBL, and so presented an excellent situation for direct comparison.
From Mountain Bluebird based on the following criteria:
- These individuals were similar in size.
- Bill was noticeably stout, not as thin as MOBL. I did not take note of color other than dark.
- The blues (especially on the males) appeared as a deep ocean blue, rather than the smoky sky blue of MOBL.
- Males (2): The rufous on the breast appeared bright, and extended around the wing and into the scapulars and back. Male MOBL lacks any rufous on breast and back. Female/Immatures (3): More subtle, though rufous brighter than any other warm-colored MOBL I’ve observed previously.
- Bellies were blue on males, gray on females (not unlike MOBL).
- Though perched in same tree and moved with mass flock, these five individuals were grouped together.
From Eastern Bluebird:
- Males had blue bellies, females/immatures had gray bellies. Eastern should have a noticeably clean white belly.
- Males had rufous breast that extending onto the scapulars and mantle, on females/immatures this was much more subtle. Eastern should have no rufous tones on back, just evenly blue.
- I did not notice a difference in the color of the base of the bill, though I did not focus on this field mark.
- I did not take note of throat color or the color of the sides of the neck. These are two important field marks to distinguish WEBL from EABL, aside from the color of the belly.
- The Sibley eGuide to the Birds of North America (structure/plumage, illustrations)
- The Merlin ID App (structure/plumage, photographs)
- eBird (range/historical records/current records)
- Leukering, T. (2013). Warm-colored Mountain Bluebirds: an unappreciated identification pitfall. Colorado Birds, 47(4), 297-299.
- Previous experience and memory.
I have seen all three species previously, but perhaps I have the least amount of experience with Western Bluebird. I learned to bird in the east, and so I have extensive experience with Eastern Bluebirds (13 years) in the field and in the hand (banding). In recent years I have traveled the west, encountering both Western and Mountain Bluebirds. Range and habitat have largely helped me in the past. Rarely, until now, have I been in a place where all three can reasonably occur, especially in migration. This experience, in particular, has hardened my ability to separate these surprisingly similar species on plumage and structure alone.
Notes made AFTER observation
No files uploaded.
Date Documentation Submitted
10/28/2019 4:55:00 AM
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