Birds don’t move–we do.

ESA 2013 #revolutionarymigration from Ty Tuff on Vimeo.

Check out this video of Ty Tuff’s presentation at ESA 2013, one of my favorites there, which really should be called “The Greater Shearwater versus the Solar Terminator.” But these aren’t ecologist wrestling throwdowns (Sadly), and no, Arnold Schwarzeneggar does not play the Solar Terminator (Thankfully). In his talk, Tuff shows how taking into account solar frame of reference, rather than our terrestrial frame of reference, can explain figure-8 patterns of pelagic bird migration, which ecologists have not been able to easily explain.

“The weird thing about clock time is that it doesn’t mean anything biologically; it’s totally biologically irrelevant,” Tuff explained. “Time is really how fast we are spinning on the planet, and our government has come up with this linear version of it…. So, what is biologically meaningful? What I think is biologically meaningful is what’s called solar stationary time. Organisms respond to the sun. So, imagine you are just responding to the sun, and the government hasn’t given you a clock, this is probably how you are going to tell time. We often see this summed up as day length, but day length is sort of an approximation of what this might be.”

The line between night, where the Earth is blocking light from the sun, and day, where sunlight is hitting the Earth is what Tuff calls the shadow line (the more jargon-y term is solar terminator, which we’ve already established needs to be someone’s wrestling name… although “Solar” and “Terminator” are already taken.). Radiolab has a cool explanation of the shadow line. Birds cross the shadow line two times a day, at sunrise and at sunset. According to Tuff, bird movements reflect the path that conserves the most energy.

“The energy flux that you experience when you go across that shadow-line is really immense,” Tuff asserts. “So what I’m trying to get at here is that we have real, physical, tangible, in-your-face examples of things that birds could be responding to that seem much more powerful than our clock that we’ve made, or our idea of day length, or our perspective that latitude’s really the most important thing.”

Birds deflect their path of travel every time they change their flight to cross the shadow line at a perpendicular angle. Because the Earth’s tilt is changing, their direction of flight changes with respect to the shadow line a little each day. The way Tuff explained this to me is with an analogy of a kayak riding a wave. The bird is the kayak, the water is the planet, and the wave is the shadow line.

Tuff showed that migrational patterns from 22 individuals of a bird with a badass name–the greater shearwater–can be explained by their solar stationary time, which means from a solar frame of reference, we are moving, not the birds:

“Your observational frame of reference is really, really important. Right now, you are moving at a thousand miles an hour, and you are moving at different directions to the sun. So, if you want to think about something that’s responding to the sun, it’s important to think about what they might be responding to, how they might be telling time, how they might be using resources.”

Tuff plans to look at how widespread this pattern is across many migratory species.

“I used an extreme pelagic pole-to-pole migrant [e.g., the greater shearwater], but if you think generally, all animals’ migrating requires is that for a couple hours a day while they’re being active, they’re paying attention to the sun, or they’re paying attention to energy in some way. You just really have to be active at the same time of day everyday, not insist on sticking to a particular latitude.”

Tuff also plans to explain differences between long-distance migration routes (those with figure-8 patterns and those with more horizontal patterns) by adding another variable, cycle-phase location. This fourth dimension has to do with singularities, or places in a feedback loop that can’t be fully measured. The way Tuff explained to me the singularities that interest him was that on Earth, in 3D, the poles are the singularities, places where there is no down but only sideways. In five dimensions, the singularities are the solstices and equinoxes. At the solstice, the tilt of the Earth’s axis toward or away from the sun is at its maximum, and so the place receiving the most sunlight switches from going further northward each day to going southward or vice versa. Cycle-phase is the distance between singularities. I look forward to hearing more about Tuff’s research on this subject as it comes to light (pun absolutely intended).

Tuff also explains more background information on the subject in his 2011 ESA talk, and there’s also more info on his website.


Birds don’t move–we do. — 3 Comments

  1. I love this! The perspective is really unique and delightful. But, I do have a problem stomaching one of his statements. Organisms don’t simply “respond to the sun”. They have real, readily demonstrable, intrinsic, internal clocks that keep roughly 24-hr cyclical time patterns and have a demonstrated involvement in migration patterns. No government-issued timepiece necessary. Granted, these internal timepieces have evolved BECAUSE of the solar cycle, and there are certainly other factors that are important (even ones we haven’t figured out yet!). Plus, there is evidence that the internal clock changes (in ways we don’t yet fully understand) when birds are migrating, so….his ideas actually provide some hypotheses that could be tested in a chronobiology context as well. Super interesting.

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