Birds, right?  Lets get into it.  This week’s bird is the bar-headed goose.  This goose spends summers nesting around high altitude lakes in central Asia and migrates in the fall across the Himalayas into India for the winter, where it often feeds in cultivated fields and can be considered a pest.  It practices intra-species brood parasitism, in which lower ranking geese will attempt to lay eggs in the nests of higher ranking females in the flock. The aforementioned migration route is truly epic, with the bird flying as high as 21,000 feet (6,400 m) over the mountains.  The air temperature at such an altitude is around -12 degrees F (-25 C).  Studies with satellites and GPS have shown that the birds spend 95% of their time at altitudes of less than 18,976 (5,784 m) and that they will take longer routes that allow lower altitude flight through mountain passes.  The bar-headed goose can not only function in such extreme conditions, but must necessarily be able to exert itself to the utmost in order to both sustain flight and gain altitude over obstacles.  It is able to do this in part because its blood bonds more effectively with oxygen at the lower concentrations found at high altitude as opposed to sea level.  It is able to fly harder the more adverse the conditions.  I was able to find an exceedingly interesting article on how the goose manages this from the US National Institute of Health.  It is excerpted below, and I have included some charts from the same source that show migrations routes with local air pressure at various altitudes along the way.  I didn’t have time to read it all, but I strongly recommend it if you’d like to know more about the miracle of nature on which I happened to stumble this week.  The article is, as far as I read, very thorough and written in language a layperson such as your humble host can understand. 

“Therefore, it would be advantageous for bar-headed geese to make use of upslope tailwinds during ascent. However, upslope tailwinds predominate only during the day in mountainous regions, and bar-headed geese often migrate at night and in the early morning when the predominant winds travel downslope. Although these nighttime flights likely entail a greater metabolic cost than flying later in the day when updrafts predominate, the darkness should lessen predation risk (e.g., from predatory birds), the wind currents are more stable and less turbulent, and the air is cooler and will have a slightly higher density and Po2. These benefits may outweigh the metabolic costs of having to flap harder to climb to high altitudes.”

Although the excerpt from NIH refers to nighttime air as having a slightly higher density, another source I found said the density change at night is equivalent to several hundred meters of altitude, and every inch counts when you have to climb four miles into the sky.  
The bar-headed goose is frequently kept in captivity and breeds regularly there.  Due to escapes (go, goose!), migration, and possibly deliberate introduction, it is becoming increasingly common in Great Britain, although rare bird sighting data are several years old.  Suspected escapees have also been seen in Florida, although they have not established an independent local population there.  

Keep it real, folks!

Links: https://tinyurl.com/mr3eue4b, https://ebird.org/species/bahgoo,

NIH paper: https://tinyurl.com/2vhkrvry