Terrestrial locomotion of the Svalbard rock ptarmigan: comparing field and laboratory treadmill studies

1. What they did

Much of what we know about how birds walk and run comes from laboratory treadmills, which give a uniform, hard, grippy surface. Wild birds move over grass, mud, rock, snow and ice. The authors asked whether treadmill results hold for birds moving in the wild, using the Svalbard rock ptarmigan, one of the few birds with a full treadmill dataset on how they move and how much energy that costs.

Ptarmigan can use three gaits:

• Walking: at least one foot is on the ground at all times.
• Grounded running: an in-between gait with a running-like rise and fall of the body, but no moment when both feet are off the ground.
• Aerial running: both feet leave the ground in each stride.

In spring 2017 and 2018, the team travelled between sites by snowmobile in the Adventdalen valley on Spitsbergen, Svalbard, and filmed wild male ptarmigan moving across level snowy ground. Only males were used (identified by their calls and red combs above the eyes) so the results could be compared with the treadmill birds. Each bird was filmed from a fixed position, at slow and fast speeds where possible, and a 1 metre scale bar was then laid along the bird’s path to work out speed. Each bird’s location was marked with the Global Positioning System (GPS) so no spot was used twice.

From 165 videos of 91 birds they measured stride length, stride frequency, how long each foot was on the ground (stance) and in the air (swing), and the share of each stride the foot spent on the ground (duty factor). They confirmed each gait by tracking the bird’s centre of mass in the video. They recorded the snow hardness for each trial, and then repeated the analysis using only birds on very hard snow, the closest match to a treadmill.

2. What they found

Gaits and speeds

• Walking ran from 0.26 to 0.91 metres per second. Running gaits ran from 0.92 to 2.76 metres per second.
• Grounded and aerial running overlapped between 1.46 and 1.50 metres per second, slightly slower than the switch suggested in the treadmill study.
• Birds walked in 48 trials, ran with grounded running in 56 and ran with aerial running in 61.
• Aerial running happened only on firmer snow: there were no aerial running trials over soft or medium snow.

Field versus treadmill

• Grounded running: wild birds took faster, shorter steps than treadmill birds at the same speeds. Stride length rose less with speed in the field, stride frequency rose more, and swing time shortened with speed in the field but not on the treadmill.
• Very hard snow only: when only birds on very hard snow were analysed, the grounded running differences disappeared, and walking and aerial running still matched the treadmill.

Speed choice

• Wild ptarmigan seemed to prefer walking at about 0.7 metres per second and, to a lesser extent, aerial running at about 1.7 to 1.85 metres per second.
• They rarely moved very slowly (0.1 to 0.4 metres per second) or very fast (over 2.1 metres per second).
• There was no clear preferred speed when grounded running.
• Their top speed in the wild, 2.76 metres per second, was 1.4 times the 2 metres per second reached on the treadmill.

Behaviour in the field

The birds walked over all kinds of snow, from soft powder to hard snow, as they moved between feeding sites, mostly at the faster, more energy-efficient walking speeds. Svalbard ptarmigan feed by pecking at the ground and scratching away snow with their feet to reach plants, mainly at sites loosely covered in snow.

3. What they concluded

• Treadmill studies give meaningful information about how wild birds move, but the surface underfoot matters.
• Walking is not affected by the surface because moving slowly cancels out its influence. Aerial running is not affected because the birds use it only on firm ground, which is like a treadmill.
• On uneven, slippery snow and ice, when birds want to go faster but the surface is too soft for aerial running, they use grounded running with faster, shorter steps. Keeping a foot on the ground and the body low probably helps them stay stable and recover from slips.
• The springy, bouncing action of aerial running seems to work only on firm ground; soft snow would soak up the energy.
• Wild ptarmigan appear to choose speeds that keep the energy cost of moving low, with top speeds probably kept for escape over short distances.
• Treadmill studies underestimate top speed, because speeds must be held for 5 to 10 minutes to measure breathing gases. Filming treadmill animals up to their maximum speed, even briefly, would allow comparison with wild animals.
• It is not yet known whether birds judge the surface by feel as they move or in advance, perhaps by sight. Bio-logging studies of gait use through the day and seasons would help.

6. Other technologies, skills or innovations

• Video recorded at 25 frames per second (Sony Handycam, 2017) and 100 frames per second (Sony Cyber-shot, 2018), with the camera held level with and parallel to the moving bird.
• A 1 metre scale bar laid along the bird’s path after filming, to calculate speed.
• Video analysis in the free Tracker software (Open Source Physics), including estimating the centre of mass from a polygon built on body points (neck or eye, base of the tail, and breastbone).
• Snow classified by hardness using an international snow classification.
• GPS-marked bird locations to avoid filming the same spot twice.

7. Implications for conservation (from the paper)

The authors say real-world data on how animals move over natural surfaces are needed to place laboratory results in an evolutionary context and, for example, to judge the possible impact of environmental change.