Date: 11/21/17 9:33 am
From: DAVID A LEATHERMAN <daleatherman...>
Subject: [cobirds] Basemar Trap in Boulder (Boulder), etc.
There are many reasons to chase interesting or rare birds. Most of mine are no different than anyone else's: add to a list of one kind or another, increase experience and thereby improve ability to find said species on one's own, enjoy the thrill of laying eyes on a beautiful or charismatic bird, nostalgia related to a species that means something in your history, etc. More and more I like to chase the sites to see if the secrets of their attraction to a notable bird can be unraveled. It seems knowledge of these sites might improve our ability to find others in the future. Yesterday I went to Boulder hoping to see some of the recent goodies, yes, but equally wanting to decipher the elements that held these birds long enough for them to be discovered by birders. We have coined the term "trap" for locations that hold birds, usually migrants, for extended periods of time while these individual birds refuel and fatten up for the rest of their journey.

My MO on "site chases" tends to be the same: give the bird a chance to get away, then go check it out. If you actually see the bird days after first reported, you perhaps extend the knowledge of its visit which helps others, but mostly you can learn about its site without being accused of "messing things up". That is, nobody can say your tromping was the disturbance that drove the bird off. If the bird is gone, the big drawback is that you can't watch it do its thing, which, of course, would take away the guesswork related to how it was using the site.

The woodcock(s) at Bobcat Ridge Natural Area in Larimer is/are a good example of what I mean here. Like zillions of us, I thrilled at watching my first Colorado woodcock rumba in the mud. And after Ranger Carl and birder reports seemed to confirm the bird had left, I went back, sampled the muck and learned a ton about what a Colorado woodcock might eat. This was chronicled in "The Hungry Bird" (April 2015 issue of "Colorado Birds, v49(no.2)).

I intended to do the same with the Purple Sandpiper situation but came up with a bunch of excuses why going all the way back up there wasn't reasonable. If the Bushong Brothers find another, I'll do better.

Back to Boulder. Yesterday I checked out 1) Skunk Creek near Basemar Shopping Center, 2) Twin Lakes (Twin Lakes Drive e of 63rd Avenue) and then 3) revisited the office park west of Foothills Parkway (3005 Center Green Drive) just n of Valmont where the Bay-breasted Warbler entertained us in Nov-Dec 2013 (how could it have been 4 years ago already!?). Here's what I found.

1) Skunk Creek - it seemed clear the main attraction was that nice little stream with a decent flow. The rest is gravy: sloped banks, heavily shaded, complex understory with lots of leaves to flip over to reveal worms and other associates of decaying vegetation, diverse woody plants with berries and insects. Chuck Hundertmark has a great photo on eBird which shows the black-throated blue with a buckthorn berry in its mouth. It appears to be European buckthorn (Rhamnus cathartica), a plant that would seem to be unpalatable but which a lot of birds are known to eat. A few berry-laden buckthorns were evident yesterday, as were midges flying over the creek, other aquatic insects like mayflies, leafhoppers on the dominant tree (crack willow), wild roses with rose hips, snowberry shrubs, green ash volunteers, a Siberian elm full of Asian bark beetles, and chokecherry bushes (fruit long gone but this tree can have aphids). I did NOT see the varied thrush or the two species of black-throated warblers (blue or gray). I did see Black-capped Chickadees and one solitaire. In summary, a gem of habitat squeezed by urban humanity and not treated very well in terms of trash, but one certainly one worth frequently checking. And I enjoyed the nature art decorating the bike trail bridge under Broadway.

[cid:4674d0eb-9c4f-4003-8ffd-a2942f5888eb] [cid:17ee0035-45c5-4e36-a44e-046754fc9bb7] [cid:19cf6f35-c0ff-4c39-854c-1188eea864e9] [cid:4eaa8d8f-77a4-4bf9-8ae9-200c659a3301]

2) Twin Lakes neighborhood - infestation of needle aphids in the Austrian Pines (Pinus nigra), probably Eulachnus rileyi. One of photos in Leslie S.'s original eBird report on November 17 clearly shows these on needles near the Tennessee Warbler. I collected several winged and wingless forms of this aphid from needles of Austrian pines near the tennis courts gone wild (just e of Twin Lakes Drive where it crosses the foot trail). I believe these same trees, or others nearby, also hosted a Nashville Warbler of late.

[cid:8b8c6af5-6997-426f-8387-e5f7084c32ae] [cid:a129245d-f56e-474e-9ab3-c23437922d2a]

I did not see warblers at Twin Lakes but did see Black-capped Chickadees in the pines. Many of the late warbler reports in Colorado (see Brandon's recent report from Pueblo) mention rare songbirds being in mixed flocks of other birds, usually chickadees. Homing in on chickadees at this time of year and searching thru them well would seem to be a good recipe for finding a tardy warbler.

3) Center Green Office Park continues to have many Austrian Pines infested with the same aphids as the Twin Lakes site, as it did when the bay-breast and other warblers (Northern Parula and yellow-rumps) pigged out in 2013. No warblers during my brief search yesterday but I did see three Mountain Chickadees and a Black-capped Chickadee. One can see these trees are infested before even getting out of the car (needles glistening with honeydew excrement of the aphids, yellow jacket males feeding on aphids and honeydew as their "last request" before being sentenced to death by sustained freezing temps).

Brandon and others no doubt have the historical Pueblo sites covered. What's going on warblerwise in the pines and oaks on the CU East Campus which has hosted Pine Warblers in past winters? Denver Office Park? Fort Logan Cemetery where Doug Kibbe had a Pine Warbler this fall? The Animas River hatchery in Durango? Undiscovered traps in downtown Grand Junction? Pines with aphids would seem to be key, but so is open water (water treatment plants, springs, etc.), recent migrant or wintering sapsucker sap-welling, midges and scale insects, even berries and feeding stations with suet.

Chase those sites.

Dave Leatherman

Fort Collins

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