I almost responded to this issue yesterday when Gregg brought it up, and I will now.
Sub-freezing temperatures are rarely an issue for birds unless they are extreme, take away the availability of free water or coincide with conditions that got a bird's feathers wet. When feathers are dry the literature says they can result in as much as an 80-degree difference in air temp compared to that under the feathers next to the skin. In other words, it could be -20 outside and 60 right next to the skin. That data comes from the famous ecologist Ernst Mayer's work in Maine with thermistors and a golden-crowned kinglet. Cute little birds get counted every year on the Nome, Alaska CBC. Ducks move on not because the water is cold but because the surface freezes and takes away their ability to tip or dive and obtain food. As Brian said, the biggest issue with birds and the onset of significantly colder weather is generally decreased food availability (and the issue of liquid water). A lot of the birds we get excited about seeing in early winter are neotropical migrants that for whatever reason are still passing thru or even lingering. These late insectivores still rely on insects, especially insect types with chemistry that suppresses their freezing temperature (usually involving glycols) and insect cadavers. Also, most neotropical migrants also have dietary flexibility that allows exploiting fruits. and when all else fails, can lower their standards and engage in desperate measures like scavenging and stealing. Midges, such as what the Bay-breasted Warbler at the Fountain sewage plant must be getting, or aphids and scales (Tennessee, Black-throated Green, Pine, Northern Parula and Black-throated Blue lately along the middle Front Range) make up the bulk of what cold weather insectivores can still find and rely on. But we also have species of grasshoppers that overwinter as adults, and even certain moths and butterflies, flies, ichneumonid wasps, leafhoppers, etc. can be active in temps as low as 45 degrees. Then there are always back-up foods like the contents of galls, cocoons, spider webs and egg sacs, feeders with suet, etc. Birds are simply amazing in coping, of course, within the limits all life forms have. It was this time of year in 2013 when we had 10 species of warblers on the Colorado RBA. One of them was a Northern Parula in Boulder that along with the famous Bay-breasted and some yellow-rumps was sustaining itself on aphids from one particular Austrian Pine. I took pics of it on 16November2013 but believe it made it into early December. Apparently none of us entered this individual into eBird if Gregg says there are no records from the Denver to FC area for early December. That brings up another misconception, that being the notion eBird is a complete record of all that is known. Far from it, but I'll leave it at that. Anybody checking the Denver West Office Park? Interlocken? 28th e of Baseline in Boulder? Bow Mar apartment complex in Littleton? Longmont neighborhood e of Hover s of Nelson? No doubt many other aphid-scale-midge "food courts" like the one Joey just found near Chatfield exist out there. Dave Leatherman Fort Collins ________________________________ From: cobirds@googlegroups.com <cobirds@googlegroups.com> on behalf of Bryan Guarente <bryan.guare...@gmail.com> Sent: Wednesday, December 2, 2020 7:57 AM To: Brandon <flammow...@gmail.com> Cc: Cobirds <cobirds@googlegroups.com> Subject: Re: [cobirds] Re: Northern Parula in December Gregg and others, I would love to hear whether the Parula and Pine Warbler stuck around after the cold frontal passage last night. I doubt the temperatures would be the problem. The snow could cause food gathering issues. The winds are appropriate for migration southward, but I don't know whether the "need" to migrate is still there. Hormonally, There must be a point in every bird that they don't feel like they need to migrate anymore. These birds will be interesting case studies from both the perspective of Dave Leatherman and from Bryan "Weatherman". Bad joke... moving on. Keep us informed. Thanks, Bryan Bryan Guarente Meteorologist/Instructional Designer UCAR/The COMET Program Boulder, CO On Wed, Dec 2, 2020 at 2:04 AM Brandon <flammow...@gmail.com<mailto:flammow...@gmail.com>> wrote: Pueblo County is up to 17 species of warblers now thst have been seen during December to February. Pretty amazing total. Most have been since the 1990s. Brandon K. Percival Pueblo West, CO Sent from my Android On Tue, Dec 1, 2020, 6:44 PM Gregg Goodrich <gregggoodr...@gmail.com<mailto:gregggoodr...@gmail.com>> wrote: There are no previous December eBird records of Northern Parula in the Denver/Fort Collins area until today December 1st, 2020. The Chatfield bird was seen and photographed today. The Springs, Pueblo, Pitkin Cnty and Garfield Cnty have had December records. It will get down to around 20 degrees tonight with chance of snow. Wonder if it will be here tomorrow. Gregg Goodrich Highlands Ranch -- You received this message because you are subscribed to the Google Groups "Colorado Birds" group. To unsubscribe from this group and stop receiving emails from it, send an email to cobirds+unsubscr...@googlegroups.com<mailto:cobirds+unsubscr...@googlegroups.com>. 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