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New Bird Migration Secrets Revealed


Those who have seen the Illustra movie Flight: The Genius of Birds recall how the migration path of the Arctic tern was revealed by the use of data loggers attached to the birds’ legs. This technique, pioneered by Carsten Egevang, has now been tried on many other species. Here are some of the new findings, as well as new details about bird design in general.

Night Flight

A bird with a peculiar name, the “nightjar,” was tracked using data loggers by scientists at Cornell and at Lund University in Sweden. Since this bird migrates at night, the researchers were eager to learn what cues the species uses to stay on track. A major finding was that the bird uses a “lunar calendar” to inform its routing and timing. New Scientist reports:

Until now, there was little evidence of a connection, though nightjars were a natural choice to look for one. The birds feed at night, snatching insects from the air. We already knew that they change their feeding habits based on the moon’s phase, gobbling more insects on bright, moonlit nights. Could nightjars also be scheduling their migration in a similar way?

To find out, Gabriel Norevik at Lund University in Sweden attached tracking devices to 39 European nightjars. Some of these devices measured the birds’ position using GPS, while others tracked their acceleration. This allowed Norevik and his team to record location over the year and flight activity levels night after night.

Their results reveal a key role of the full moon in the nightjar’s itinerary, which consists of long night-time flights with daytime resting punctuated by much longer rests at stopover sites. On moonlit nights, the birds’ foraging during migration stopovers more than doubled. [Emphasis added.]

The research paper, published in PLOS Biology, concludes:

European nightjars clearly exhibit periodically fluctuating activity patterns on both daily and seasonal scales that are strongly associated with the lunar cycle and likely driven by light-dependent foraging opportunities.”

How this sensible strategy might have originated is not stated. In fact, the paper says nothing about evolution. What’s remarkable is that these birds know how to use the moon to travel all the way from northern Europe to sub-Saharan Africa, even in the dark or in dim moonlight. Clearly there is more to learn: “Incorporating temporal dynamics of relevant environmental factors, such as celestial bodies, could thus be a promising inroad to further our understanding of the seasonal pulses of animal migrations and their trophic effects.”

Sea Bird Fly

Another species tracked with data loggers is the Manx shearwater, a handsome foot-long seabird with a 30 inch wingspan. It looks like a flying cross with its dark top and white underside. These birds are among the most long-lived of birds, able to live 55 years. They live in many places around the world, but predominantly in the British Isles. They have to travel to small islands to feed and breed, so scientists at the University of New South Wales (Australia) were curious to know how they do it. Their results are published open-access in PNAS with a title that explains the results: “Shearwaters know the direction and distance home but fail to encode intervening obstacles after free-ranging foraging trips.” They use a minimal-cue strategy:

Procellariiform seabirds homing from distant foraging locations present a natural situation in which the homing route can become obstructed by islands or peninsulas because birds will not travel long distances over land. By measuring initial orientation from Global Positioning System (GPS) tracks during homing, we found that the Manx shearwater fails to encode such obstacles while homing, implying a navigation system that encodes the direction of home rather than a learned route. Nonetheless, shearwaters timed their journeys home, implying that their navigational system provides them with information about both direction and distance home, providing evidence that for routine, yet long-distance navigation, seabirds probably ascertain homeward direction by comparing their current position and the location of home with 2 or more intersecting field gradients.

When facing obstacles in their path, do the birds deviate from the Great Circle (shortest route)? The data show that the birds don’t need to care about obstacles in the way; they just point in the right direction and give themselves the expected time and fuel to get there. But what happens when they encounter obstacles, like land that they prefer to avoid? Whatever they are doing, it works, because they usually arrive at the right place on time. 

The scientists ruled out path integration (like ants use) or memory of previous flights. 

These findings support a third possibility: that shearwaters might be using true navigation to calculate their location relative to home by comparing the current values of (at least) 2 intersecting environmental gradient fields, the characteristics of which they had learned in their familiar area closer to the colony…. While it remains possible that shearwaters could use more than one of these mechanisms, our findings are probably the clearest evidence to date that a wild bird uses gradient-based navigation to home and that this has map-like properties.

Amazing: birds do math. They “calculate” their way home from two gradient fields. This will send scientists on a new hunt for how they  do it: employing “specific cue-use underpinning map and compass navigation” in their bird brains.

The Landing

When a bird arrives home, how does it land? Watch a parakeet hopping around its cage from perch to wire to mirror to perch. That’s incredible. Birds can quickly land on limbs and objects of different sizes and shapes. How do they do it? 

Writing in Nature, Andrew Biewener attempts “Getting to grips with how birds land stably on complex surfaces.” Lab experiments with parrotlets (that look like the familiar pet parakeets) showed that they flex their toes wide open as they see the target perch, then quickly clamp down on it. 

Tree-dwelling birds can land on perches that vary in size and texture. Force measurements and video-footage analysis now reveal that birds rely on rapid and robust adjustments of their toe pads and claws to land stably.

There’s more to it than the toes, because the bird has to know its center of gravity; otherwise it might somersault around the perch. It also has to use its wings to decelerate appropriately. Biewener began with praise for the graceful landings of flying animals:

Even casual observations of flying birds, bats and insects reveal the adept and seemingly effortless ability of these creatures to land and take off safely from a wide variety of surfaces, whether these are tree branches, telephone wires, flowers or rocks. By contrast, passenger aircraft usually require long, flat runways to accomplish the same feats, and, even so, accidents can occur during take-off or landing. With the rise in the use of aerial drones for a range of applications, and the challenge of improving the aerodynamics and energy efficiency of drones, given their small size, there is interest in developing drone design to boost their success in landing on a range of complex surfaces.

Once again, we see that intelligent design in engineering often looks to nature for inspiration. These three evolution-free studies show also that the sheer love of natural design is a stimulus for good science. When one sees a magnificent accomplishment, whether flying thousands of miles in the dark or landing gently on a round perch, science wants to know how it was done. No Darwin required.

Photo: A nightjar, by Dûrzan Cîrano [CC BY-SA 3.0], via Wikimedia Commons.