John et al,
First off, we can't know the answers to your excellent questions. Here are my guesses.
More, maybe way more, "off-course" individuals than we ever detect pass through our 'midst. I've heard many birders wish, upon leaving a site, they could get a printout of what really is present vs. what we put on our checklists.
Birds watch other birds the way birders watch other birders to detect something they are after. "Hey, those guys are scoring, maybe I can."
Feathers are wonderful insulation if they can be kept dry. As such, cold temperature, as it directly affects migratory birds, was probably not the evolutionary impetus for these species to initiate travel between breeding and wintering sites. The ecologist Berndt Heinrich measured a difference above (ambient air) and below the feathers (next to the skin) of a Golden-crowned Kinglet in Maine. It was 80 degrees! Minus 20 outside, 60 at the skin.
Cold temperature, as it affects the availability of food and unfrozen water, was/is probably the most important indirect driver of migration.
Birds are quite resourceful. Insectivores often eat berries. Chickadees eat fat from an elk carcass. Boulder Creek warblers prefer caterpillars and live aphids, but a moth pupa under loose bark, frozen aphids, a green lacewing and a daddy longlegs will do in a pinch.
Colorado's hotline or rare bird alert usually has a bunch of warbler species on it in late November-early December. Birders have long known late fall is perhaps the best time to see concentrated, "gettable" eastern warblers.
Putting this all together, my guess is the majority of late-migrating warblers we see are first-timers. They didn't get the memo or notice their brethren leaving two months ago. Maybe they are offspring of parents who for a myriad of reasons were forced to nest late, or follow-up a failed first-nesting up North. A winter weather system jolts them into taking their first migratory journey. Maybe, besides being late, their vector is off by 10 or 45 degrees. Instead of southeast, they go southwest. The first one to discover the insect (and other) food resources of Boulder Creek forages with gusto. Another off-course individual notices a member of a feeding-guild similar to its own and checks it out. Feeding with a non-aggressive cohort has its advantages in terms of predator detection, etc. Mixed-species flocks are no accident. As Vonnegut would say, "and so it goes". I don't think the Boulder Creek assortment started migration together, I think it was built one individual at a time. Will they travel together after leaving Boulder? Who knows? They might leave together, but the hearts of their historical wintering grounds are not the same. At a stopover like Boulder, the cast might grow to 10 individuals of 6 or more species. The sugar maple full of one particular aphid in Grandview Cemetery this fall attracted at least 14 resident and migratory species. Maybe a severe weather phenomenon will end it. But my guess is depletion of the food resources will be just as, if not more, important in closing the curtain on this show. And, of course, we wish them well.
We should all thank who first noticed the onset of the Boulder event this year. Who was it?
Just my take, and I could be wrong about the majority of it. Others chime in, please.
Dave Leatherman
Fort Collins
From: 'John Malenich' via Colorado Birds <cobirds@googlegroups.com>
Sent: Wednesday, December 3, 2025 9:14 AM
To: Colorado Birds <cobirds@googlegroups.com>
Subject: Re: [cobirds] Nashville warbler, Northern Parula, and Chestnut sided warbler....
I'm also quite curious if anyone has any thoughts on the underpinnings of this bouquet of eastern warblers that have been inhabiting Boulder Creek for the past few weeks. There have been 6 different species--Chestnut-sided, No. Parula, Tennessee, Pine, Orange-crowned & Nashville—holed up in this location for some time now. None of them choose to leave with this past weekend's cold snap into the low teens (which I expected likely would happen). We'll see what comes of today's snow storm.
But this has had me pondering how this group might of all come together. We're these birds all migrating together as a mixed flock and came to this location together due to the food resources they found here? Or did they separately get off course and stumble upon this Aphid-rich spot? Would such a coincidence be possible? All six of these birds are well out-of-range, considerably off their typical migration route, and quite delayed in their travels south. Are they sticking around so long because they can't get enough food and put on enough fat to fuel the next leg of their journey south or is the eating so good they just don't want to leave? Obviously, I know we likely cannot know with any real certainty how this came to be, but I'd love to hear what thoughts folks may have about gathering's origins.
John Malenich
Boulder, CO
On Tuesday, December 2, 2025 at 5:33:50 PM UTC-7 DAVID A LEATHERMAN wrote:
John et al,
I visited this area with friends on Black Friday. When in the trees, I think these insectivores are mostly getting APHIDS! Probably still a few alive but mostly I suspect the birds are getting cadavers. Aphids are high in sugars and would be highly sought after at this time of year for sustenance and migratory fuel. The birds might also be getting aphid predators like green and brown lacewings, maybe some midges when near the creek.
On a related note, the next "The Hungry Bird" article in press (published in "Colorado Birds", the publication of the Colorado Field Ornithologists) is about painted maple aphids in one sugar maple in Fort Collins and the assortment of birds they attracted this fall.
Dave Leatherman
Fort Collins
From: cob...@googlegroups.com <cob...@googlegroups.com> on behalf of John Tumasonis <snake...@gmail.com>
Sent: Tuesday, December 2, 2025 3:20 PM
To: Colorado Birds <cob...@googlegroups.com>
Subject: [cobirds] Nashville warbler, Northern Parula, and Chestnut sided warbler....
All:
I bit the bullet and decided to check out the warblers near Boulder High today (12-02-25). I found three of the five species reported near the 17th Street and bikeway intersection. Several other birders had seen 4 of 5 species, before I arrived there. Here are a few things I noted:
1) These warblers are keeping close in loose congregations - if you see one species, you'll probably see several others.
2) Nearby flocks of red-breasted nuthatches, juncos, house finches, and bushtits may or may not be associated with the warblers.
3) The warblers I saw a feeding at heights of 12 to 40 feet up in all different kinds of trees, such as, willows, ash, oak, and cottonwood.
4) Almost all the trees being foraged on had some leaves - either desiccated, or turning yellow to brown. Gleaning (something) from both leaves and branches.
5) The warblers don't just "disappear" - they are foraging on both sides of the creek and upslope into brush onto CU campus properties, and Boulder High properties. They are also moving up and down the creek drainage.
6) Reports suggest they are foraging at all times of day: 8, 9, 11, and 12 and afternoon at 3 and 4 pm. They appear very hungry and are always in motion.
If trying to take photos I would suggest 200 to 400 mm lens, with high burst speeds.
And then the big questions: Why are these birds congregating is this location at this time of year? Will they continue through the next cold front coming on Wednesday? And what the heck are they eating?
Thoughts?
John T (Tumasonis), Broomfield CO
"I'm not a real birder. I only pretend to be one on CoBirds."
