(An ashy-headed goose demonstrating its exceptional balancing ability; image from Shutterstock.com)
The other day I spent some time exploring the “arm” at Guelph Lake, curious to see how far up the Speed River I could get in my kayak before bottoming out. (Turns out, not very far. Water shoes would have been a good idea. I definitely need to try this earlier in the season when the water is higher.)
(map of Guelph Lake with Speed River indicated)
It’s a very quiet stretch of water and, as I came around a bend with a magnificent willow holding court on the bank, I paused in my paddling to drift soundlessly past a large flock of Canada geese napping and preening in its shade. Some were standing, some were resting on the ground with their heads tucked around on their backs, and quite a few were balancing nonchalantly on one leg. Why is this a comfortable resting position? If I have a few quiet minutes on a summer afternoon, I don’t feel inclined to strike a tree pose on a riverbank …. what’s up with our feathered friends?
Canada goose balancing on one leg on a rocky beach; from Shutterstock.com 
Woman in tree pose beside water at sunset; from Shutterstock.com
Canada geese are not the only bird to strike such a pose – flamingos and other long-legged aquatic birds such as storks and herons are perhaps more often pictured in this position. A recent report in Functional Ecology estimates that standing on one leg has been observed in approximately one third of the 852 avian species in their study.
In order to stay upright in this position, a two-footed creature (goose, heron, human, etc.) has to shift their weight slightly to the side to get their centre of mass over the single supporting foot, as is evident in the head-on photo of the ashy-headed goose at the top of this article, as well as in this video clip of my ever-helpful daughter, Mara, filmed for our AMASE video series: (sound on)
It takes effort for us to maintain this crucial vertical alignment of our centre of mass and the small area of support afforded us when only one foot is touching the ground. So what’s the advantage for birds to stand this way? There are two possible motivations commonly proposed by biologists: heat regulation and reducing muscle fatigue. However, it’s also possible, based on a recent study reported in Biology Letters, that it’s all about forces and torques (aka mechanics)!
First, let’s talk about the standard theories. When you see a long-legged bird standing on one foot, the retracted leg is often tucked up underneath the body, hidden inside the cozy plumage. This gives rise to the heat regulation theory: when the surrounding air is cold, a lot of body heat is lost through the legs since they are not covered with insulating feathers. Birds do adjust for this by reducing blood flow to their legs when its cold outside, as well as employing a neat heat-exchange trick between the cold blood coming back to the body from the feet and the warm blood heading down from the heart. Biologists have speculated that the one-legged pose is a means of further reducing heat loss during chilly weather by insulating one of the bare legs inside the chest feathers. But this couldn’t be the reason for my Guelph Lake geese friends, as the weather was a balmy 24°C that afternoon. And observations in the literature are mixed as to whether this pose is seen more often at lower ambient temperatures. Plus, the lifted leg is not always hidden inside the plumage; sometimes it’s only partially lifted and sometimes it’s extended back behind the bird. It seems that heat conservation has a role to play, but it can’t be the only reason.
The other theory is something you have probably done – shift your weight from one leg to another when you stand for long periods of time. By shifting your weight, you are temporarily giving the muscles in one leg a little break. Seems reasonable that birds might do the same.
(Gif of the Brady Bunch, rapidly switching weight from one leg to the other, from Giphy)
However, there is a big difference here: we don’t lift the resting leg completely off the ground into mid-air when we give it a break. This takes energy and requires more muscle firing in the supporting leg to keep our balance over a smaller supporting area. According to a thorough summary by Dr. Reinhold Necker, professor emeritus from the University of Bochum in Germany, “there is so far no experimental support” that this one-legged bird pose is struck to reduce muscle fatigue. Which brings us to the 2017 paper in Biology Letters.
The study conducted by Young-Hui Chang and Lena H. Ting (Georgia Tech and Emory University, respectively) looked at both the anatomy and the behaviour of the one-legged stance in flamingos. With cadaver samples, the team manipulated the body of the flamingo while it was held in a one-legged pose by clamps, testing the passive stability of this position. The researchers also analyzed the patterns of the pressure exerted by the foot of live birds while they stood on one foot on a force plate, both awake and asleep.
The clever combination of experimental manipulations of cadavers and measurements of exerted pressures on force plates from living subjects led the team to conclude that it may be energetically favourable to stand on one leg due to a passive, gravity-driven body weight support mechanism in the lower joints of these birds.
From the cadaver studies, Chang and Ting observed a balance between the rotation about the knee and hip joints due to gravity and a countermeasure arising from anatomical limits on the range of motion of these joints – i.e. certain bony and cartilaginous structures may act to block the joints against the destabilizing rotations that would otherwise happen. Chang and Ting observed that this anatomical countermeasure only activates when the foot is positioned a little inside the hip joint, as it naturally is when a bird (or Mara, see above) tries to stand on one leg!
The force plate data from the live subjects further confirmed that flamingos are incredibly good at passively balancing on one foot, as the point of highest pressure remained static for long periods of time, even when the birds had their eyes closed. Try standing in tree pose for a few minutes and focus on how much the contact between your foot and the floor is shifting to maintain your balance – I predict that it moves around considerably more than the corresponding observations in flamingos by Chang and Ting.
(Image from Chang and Ting, 2017, Biology Letters 13: 20160948)
As a physicist, even I realize that applying a study in flamingos to understanding an observation in Canada geese is a little suspect. But, as a physicist, I love that an everyday sight such as geese relaxing on a riverbank has led me to discover a free-body diagram in a paper in Biology Letters! Partially thermodynamics, partially mechanics, and wholly fascinating, I now have a slightly better understanding of why geese are yoga gurus. Can’t wait to share this with my first-year biological science students the next time I’m at the front of the room in PHYS*1300!
References:
Birds standing on one leg: mechanisms and meaning; Dr. Reinhold Necker, http://reinhold-necker.de/seite11a.html
Chang Y-H and Ting L. H. 2017 Mechanical evidence that flamingos can support their body on one leg with little active muscular force Biology Letters 13: 20160948 http://dx.doi.org/10.1098/rsbl.2016.0948
Pavlovic G., Weston M. A., Symonds M. R. E. 2019 Morphology and geography predict the use of heat conservation behaviours across birds Functional Ecology 33(2) 286-296 https://doi.org/10.1111/1365-2435.13233
Wonder Why 
