For Thanksgiving this year, we decided to do a road trip to Arizona and New Mexico to visit family and a few interesting birding hot spots. We knew that the weather was likely to be chilly in November but we ended up driving across Arizona into New Mexico on the trailing end of a very cold and snowy storm, much colder than typical for this time of year. The cold settled in for most of our trip though the skies did clear, providing spectacular clear blue skies and landscapes.
Snow Geese rising.
Watching birds on this trip got me wondering about why and how birds' feet and legs don't freeze when they are exposed to sub-freezing temperatures for days on end. Geese, ducks, and Sandhill Cranes even stand or float in water for extended periods of time, especially at night, to avoid predators. Makes my feet ache just thinking about it! Googling a bit, I discovered the term "rete mirabile," Latin for "wonderful net." Both terms sound a little magical to me - sort of like birds standing in a near-frozen pond all night and not ending up with icicles to stand on.
(Note - please see discussion in the comments. Several people who have studied ornithology and/or biology have expanded the discussion about rete mirabile. Their input is that this process is more about maintaining birds' core temperature than preventing their legs from freezing. Oversimplification on the Internet? Who knew?)
I found a Living on Earth BirdNote called Why Birds Feet Don't Freeze - here's the description of rete mirablile provided by Michael Stein:
Unlike our feet, birds’ feet are little more than bone, sinew and scale, with very few nerves. But it takes more than a lack of nerves to keep their feet from freezing. A miraculous adaptation called rete mirabile is responsible. This fine, net-like pattern of arteries that carry warm blood from the bird’s heart is interwoven with the veins carrying cold blood from the feet and legs. This interweaving warms the cold blood in these veins, before it reaches the bird’s heart. This system keeps the bird’s legs and feet warm, even without leggings and slippers.
There's a more general description of rete mirabile at
Wikipedia.
Kimberly Bostwick, Curator, Birds and Mammals at Cornell University Museum, adds more about other strategies that birds use to protect their legs in cold weather including tucking up one leg at a time for some warming near the body, or squatting on a perch to warm both legs when it gets extremely cold. And lastly:
There are two more things I want to say. The first is that even we scientists don't have all the answers to questions like this. Blood flow and hiding feet cannot be the whole story. Maybe the scales on the feet of the bird are less likely to frostbite than skin is. For instance, we can not get frostbite on our hair, because our hair is not actually living, much like the scales that cover birds feet. Maybe bird feet are so skinny so that there is less flesh in there to freeze!
With that introduction, on to some examples of birds surviving in the cold.
We spent two and a half days at Bosque del Apache National Wildlife Refuge, near Socorro, New Mexico. Large numbers of both Sandhill Cranes (mostly Rocky Mountain population of greater Sandhill Cranes) and Snow/Ross's Geese overwinter here so November is wonderful time to visit - just bring lots of layers for the cold.
There's a pond near the entrance to the NWR where Cranes spend the night to avoid coyotes and other predators. The first morning, we arrived before sunrise to watch them fly out to nearby cornfields to graze during the day. The flock included crane families like this one:
Sandhill Cranes are monogamous and mate for life; the parents escort their progeny from the nesting ground to the wintering area and back the following year. The "colt" can be identified by its brown cap and lack of red on its face.
Here's a crane taking off from the pond. Note the ice rings that formed on its legs as it stood in the water. Brrr...
Cranes spend their days grazing in corn and other ag fields. These cranes are taking advantage of fields managed for their benefit.
It was snowing lightly this morning - soon to be replaced with bluebird skies.
In addition to Sandhill Cranes, tens of thousands of Snow/Ross's Geese overwinter at the Bosque. The second morning we were there, we watched an enormous flock of geese rise from a pond with an enormous cacophony of squawking. The morning fly out is quite a spectacle, well worth the early rising time to observe. The photo in the Introduction, from an afternoon rising not from the morning fly out, gives a sense of the density of the flock flying around.
A pleasant surprise for me was the number of blue morph geese in this flock. Here in the Central Valley, blue morphs are fairly uncommon. I'd estimate ~5% of the flock in New Mexico was blue. I had fun picking them out for photos.
This one is more of a "light" blue morph.
A few more from the Bosque.
Juvenile Bald Eagle in the first rays of sun.
Chihuahuan Raven.
Gambel's Quail acting as a sentinel.
Red-tailed Hawk...
...taking flight.
Our next birding destination was at Sandia Crest, outside of Albuquerque, New Mexico. Blue skies continued but it was still cold so the snow had not melted, making for a lovely drive to the top of the mountain.
At the end of our drive, we were rewarded with sightings of some spectacular tiny birds surviving in this very cold, snowy, and windy place. One of my favorites is the Mountain Chickadee. There were lots of them bouncing around in the trees and at the feeders placed around the restaurant at the top. (Note - the Sandia Crest House is a birder-friendly establishment that seems to be struggling to stay afloat. Be sure to stop in for a bite or a souvenir if you are in the vicinity.)
It was also a treat to see White-breasted and Red-breasted Nuthatches, the latter being very aggressive about chasing other birds away from the feeders. Little dive bombers!
The target species for many who make the trek to Sandia Peak are the
rosy finches - Black Rosy-finches, Gray-crowned Rosy-finches, and the Brown-capped Rosy-finches. We were not disappointed! I'm still sorting through my photos figuring out who's who - the male Black Rosy-finches are easy but throw in two more species along with females and juveniles and it's surprising difficult to tell which is which. I'll share a couple to start. I'll follow up in a future diary once I get things figured out.
Mixed flock of Rosy Finches.
Male Black Rosy-finch.
Gray-crowned Rosy-finch (I think).
Not all of the wildlife enjoying the handouts at the feeders was avian. We saw very few mammals on this trip so it was fun to see this Abert's Squirrel climbing the pole to the feeder!