Multi-cored vortices support function of slotted wing tips of birds in gliding and flapping flight

What is it about?

In many bird species, the feathers that form the tips of bird wings are shaped such that they separate from each other when the wing is spread. You can see this very clearly in thermal soaring birds, but many other birds (such as jackdaws) also have this. So we measured the flow around the wing tip of our jackdaw and found that each feather produces its own little trailing vortex, at a different vertical position than its neighbours. This pattern fits with how we think these slotted wing tips help reduce drag. What we found particularly interesting is that this pattern also showed up in flapping flight. Many bird species have slotted wings, while only the largest are typical soaring birds. On the other hand, there are gliding specialists, such as albatrosses, that do not have slotted wing tips. Instead, these birds have extended their wingtips, which has a similar drag reducing effect in gliding flight. Therefore we think the slotted wing tip may actually be an adaptation for flapping flight.

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Why is it important?

Slotted wing tips are typically viewed in the context of large thermal soaring birds. This is not surprising, as the separation is much easier observed when the wings are not moving. However, this also results in a false impression that it is these large soaring birds that evolved this trait. First, our study provides the direct observation in a free flying bird of the aerodynamic mechanism that allows slotted wing tips to reduce drag. This supports previous claims about this function of the separated primaries. Then, as we find the same pattern in flapping flight, we argue that the slotted wing tip is not uniquely beneficial to soaring (gliding) birds and it may even have been that flapping flight has driven the shaping of these specialised feathers.