Quadrupedal Launch in Birds

Below is an excellent slow motion video from Ken Dial's lab. It shows two mallard ducks (of differing ages) launching from water. Those that have read my work previously know that I have hypothesized quadrupedal launch as a takeoff strategy in pterosaurs from land and water. What is often overlooked is that birds have also evolved quadrupedal launch multiple times, but only in an aquatic context.

In the video you'll see the duck use the wrists and outboard wing to generate much of the initial launch from the water surface (including the escape from surface tension, which is a key component). The feet then follow to complete the quadrupedal push against the water.

Animal launch is mostly ballistic, to the extent that even flying snakes have evolved a method of leaping (using the tail). Among birds, wing-involvement in launch has evolved at least three times, with multiple origins in ducks and one origin in "falconiform" birds (specifically, osprey). It also appears to be used by sea eagles and some other water birds, but has been investigated in less detail in those animals. Water launch using the wings is also reported for fishing bats of the genus Noctilio. Launch strategies from water among living animals include single leaps (like the mallard), saltation (example: pelicans), and running into a leap (example: grebes).

There are four known reasons that launching animals use leaping to takeoff: clearance, speed, efficiency, and burst acceleration. The relative importance of these different components varies according to the size and anatomy of the animal, and the substrate from which it is launching. Circumstances related to prey capture or predation may also affect the details of launch (for example, hummingbirds change the relative force input from their wings and feet when launching under duress). 

Clearance and speed are self-explanatory. The advantages of greater starting acceleration (and thus shorter launch times) are also relatively straight-forward. For some animals, these high accelerations may also improve starting vorticity, though that might be a minor advantage for larger, faster animals (see conversations between myself and +Lorena Barba, along with her students here and on her blog).

The efficiency angle may not be as obvious, however (at least, it wasn't as obvious to me when I was first learning this stuff as a student). As it turns out, lift-based propulsion isn't very good "out of the gate". At very low starting speeds, drag-based propulsion is considerably more efficient. This is why fish use the sides of their bodies to fast start, and then switch to their tail propulsion (which is lift-based) after they accelerate. Vogel (2003) provides a great explanation of lift vs drag starting from zero speed. In short, thrust from lift starts relatively low at near-zero speed, and then climbs as speed increases. At very high speeds, it drops off again until maximum speed is reached (constrained by advance ratio). Thrust from drag, however, starts out very high right at zero speed - it is therefore the better fast-start mechanism, which makes it the better launch start mechanism.

Reference: Vogel, S. 2003. Comparative Biomechanics : Life’s Physical World. Princeton: Princeton University Press.
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