Dawn Chorus: Baby, it's cold outside!

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.