I visited the Boulder site of the recent rare bird complex to try and see the birds, of course, but also to try and figure out the "why" part of it. The block north of Baseline between 29th and 30th certainly has understandable bird attractions in the form of a flowing creek with rich riparian vegetation in both the understory and overstory, plus landscaped deciduous and coniferous trees. That said, I could not really figure out anything obvious about why the warblers are there other than the usual late fall-early winter situation with pines that harbor a few species of aphid that have been the focus of late-moving insectivores in the past. I checked the pines, and also the big crack/golden willows along that creek near the red shed in which the warblers have been reported, and did NOT see any aphids. They are probably there, mostly in the upper pine crowns, maybe settled down into the base of needle bundles. They were not visible on lower branches out on the needles as has been the case in the past at Pueblo City Park, Denver West, Longmont Meadowbrook neighborhood, and other such "hotspot" sites at this time of year on forward into early December. It wasn't too many years ago when we had 10 species of warblers on the Colorado RBA in early December, all concentrated in the same sort of pines-with-aphids sites, but also including buckthorn berries (Lakewood Prothonotary at Centennial Park). On my visit to Boulder on 24October I did not come across any of the special warblers but I notice they have been seen since, so something is holding them at that site. Please, somebody figure it out. The only thing I noticed was on willow branches with leaves attached: fairly significant defoliation damage by one of the willow leaf beetles (Chrysomela knabi). Not sure which stage of this insect might be available at this time of year for birds to eat but maybe adults (look somewhat like ladybird beetles in terms of size and color pattern, yellow or orange with black spots, see below), or maybe larval cadavers.
Adult willow leaf beetle (Chrysomela knabi), actual size about 3/8 inch long. Damage is ragged chewing (adults), shotholes (larvae).
As for the Wood Thrush, it has lots of options from the berry aisle. I have looked at images accompanying all the eBird Wood Thrush checklists from Boulder of late and two things keep showing up in narratives and images: crabapples and mountain-ash berries. The crabapples are what I saw it going after repeatedly on the north side of the creek just w of the footbridge w of 30th. Gwen Moore had photos on her checklist of it eyeing mountain-ash berries, also just w of the footbridge just w of the aforementioned crabapple on the north side of the creek. Photos of both are below. I also saw plenty of common buckthorns in the area loaded with dark blackish-purple fruits, a favorite of thrushes. Last Friday up here in Fort Collins just e of Grandview Cemetery I had a late Swainson's Thrush feeding on honeysuckle berries and a prominent buckthorn grew right next to the honeysuckle which I would wager got sampled, also. A Hermit Thrush has been along the ditch in the cemetery in a large buckthorn for the last two weeks.
Would be interesting to see how long the rare bird complex persists in Boulder. Insects loaded with anti-freeze chemicals that suppress their freezing temperature, and fruits, should still be available to sustain the birds, as would cadavers of insects killed by the snow and cold. Figuring out food habits is often easier when weather conditions are bad because the birds are in desperado mode and place food acquisition ahead of worrying about potential threats like approaching birders to the extent they would when not so stressed from a nutrition standpoint. Priorities!
Mountain-ash berry clusters (left), small fruited variety of crabapple (right)
Dave Leatherman
Fort Collins
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