Friday, September 19, 2014

Why Do Birds Bob? The Way You Move Ain't Fair, You Know.

     I recently had the pleasure of watching an Ovenbird, a Swainson’s Warbler and a Northern Waterthrush feeding on the same patch of ground as fall migration heats up here in South Florida. The feeding techniques of the three birds are distinct. The Ovenbird struts about like it owns the place and picks up food along the way. Swainson’s Warbler hops and scratches, always staying hidden under one branch or the other. The Waterthrush scratches too, but shows a characteristic bobbing motion of its nether parts as it moves about. And that is the subject of this note.

     There are several birds that bob as they walk, or even while standing. In North America the Spotted Sandpiper comes to mind. But there are others too. The waterthrushes, American Dipper, and Solitary Sandpiper all do it.  The biggest bopper of all is the Wandering Tattler. World-wide other related birds bob. Bobbing is not to be confused with tail flicking. Wagtails and pipits mostly tail-flick. So do a number of flycatchers. Bobbing involves moving more than the tail feathers.  Why birds do this has never been determined, and rarely studied, even though many have commented on it in the literature. There are lots of explanations:
  1. It flushes insects as they walk.
  2. It’s camouflage, as most of these birds spend time near running water. Predators have trouble seeing the bouncing bird next to the babbling water .
  3. It helps the bird visually, like head nodding in a pigeon.
  4. It’s signaling to a mate or companion.
  5. It’s territorial dominance display.
  6. It burns off nervous energy.
  7. It helps digestion by moving the food along the gut.  Like going for a jog after Thanksgiving dinner. 
  8. It’s a predator deterrence signal, telling predators  “I am so fit that I can waste effort doing this. You better go chase someone weaker”. Thompson’s Gazelles leap into the air (pronking) for the same reason.

All of these explanations have shortcomings.
  1. These birds eat a lot of buried prey that doesn’t respond to flushing.
  2. Any predator that can’t tell a bird from water needs to become a vegetarian.
  3. The heads in bobbing birds remain perfectly steady and are not affected by the bob at all.
  4. With the exception of wintering pipits, these birds tend to be solitary.
  5. This explanation is more believable, as juvenile birds are less vigorous at bobbing . A video by Don DesJardin shows an adult Spotted Sandpiper bobbing while tussling with a younger bird, who is not.
  6. This is absurd, as new hatchlings bob from day one. They have no energy to waste.
  7. I couldn’t find enough Scolopacid gastroenterology literature to investigate this.
  8. This is an attractive hypothesis, especially considering that the rate of bob increases as danger approaches. It does not explain why Spotted Sandpipers (video by Hector Ceballos-Lascurain) and Wandering Tattlers (video by Don DesJardin) feeding on the beach bob little, if at all.
      I endeavored to find alternative explanations, and so began reviewing my video of various shorebirds for clues to why some of them bob, and others do not. It quickly became apparent that bobbing happens with a slow walk, but not with a run or a dash. Bobbing almost always accompanied picking ones way through sticks, trash, over rocks or logs, and wading in shallow muddy water. Not so much on the beach, and certainly impossible while immersed at dowitcher depth. Bobbing in both Spotted Sandpipers and Wandering Tattlers involves the whole torso and occurs at a rate of 2-3 per second. Activity so energy consuming must be very important.

      Slow motion video of a Spotted Sandpiper revealed that at the top of a bob, the rearmost leg is pulled up with the body, after which it is carefully moved forward. Could bobbing be a means of preventing trips, stubs and possibly worse injury while walking on uneven terrain where grass, mud or dirty water prevents seeing where you are stepping? Snagging a toe on a rootlet could be dangerous. Assuring that your foot is fully out of the soup before trying to move it forward sounds like a good idea. Think of how you walked the last time you tried to negotiate a mangrove swamp or river bottomland. You raise your feet all the way out with each step. Think of the injured birds you have seen. Many of the injuries are foot injuries. Anything that could lessen this would be of value.

     This reflex may have added benefits while in thick mud. The constant up and down motion transmitted to the feet makes it less likely that toes become mired in tenacious muck. When the neighborhood Merlin is in a stoop at you, the split second gained by having your feet relatively free at takeoff can be a lifesaver.

        WanderingTattlers (video by Don DesJardin) don't fit the Spotted Sandpiper model. Their bobbing is more constant, steadier in its amplitude and doesn't seem to affect the movement of the legs. This very old video of a tattler walking in a creek gives the impression that the bird is doing all it can to maintain its footing on the jagged stones. Could tattler bobbing be related to balance? The idea is not that far-fetched. If you have ever played with a gyroscope, or repaired a bicycle wheel, you know that a rotating wheel tries to maintain its vertical orientation. This is why you don't fall over when you ride a bike. A bobbing tattler is not a wheel, but it is a pendulum. It swings back and forth with an amplitude of about 6 centimeters, at a rate of 2 swings per second. The fulcrum (pivot point) is at about the lesser coverts, making it more like a seesaw than a sideways pendulum (as the tail goes up, the “shoulders” go down). The resulting movement creates angular momentum, the force that keeps the bicycle wheel steady. Yes, it must be very small. This is a bird, not a bicycle. But a small steadying force might give the tattler just the stability it needs to find its footing and negotiate treacherous terrain. It is of interest that in videos, Wandering Tattlers feeding on sandy beaches do not bob much, if at all. Balance then on a rock, or put them in more uneven ground and the bobbing is easy to see.

        I certainly don’t advance these speculations as a definitive explanation for bobbing behavior in birds. They do, however, point out once again, what you can learn about birds from watching video.