Reading: Diagram Label Completion: Question 5

Most birds produce a rushing or whooshing sound as they fly, generated where fast-moving air spills off the wing's trailing edge and where feathers rub against one another during each wingbeat. Owls, however, are able to fly and dive almost silently, an ability that depends on three separate structural features rather than on any single adaptation.

The first feature is found along the wing's leading edge, the forward-facing border that meets the oncoming air first. In most birds this edge is smooth, but along an owl's leading edge, the primary feathers end in a row of stiff, comb-like points known simply as the comb, which breaks up the large, noisy vortices that would otherwise form as air rushes over the wing into a mass of much smaller, quieter ones.

The second feature lies at the opposite side of the wing, the trailing edge, the rear border from which air finally leaves the wing. Here, instead of ending in a stiff, well-defined line, an owl's flight feathers taper into a soft, irregular fringe, which breaks up the airflow gradually rather than letting it separate abruptly, the main source of the rushing noise in an ordinary bird's wing.

The third feature covers the wing's upper surface, which in an owl is coated in a layer of fine down. This down absorbs high-frequency sound rather than reflecting it, and also reduces the friction noise produced as adjacent feathers slide across one another during a wingbeat.

Together, these three features shift the noise an owl's wing does produce down to frequencies below around two kilohertz, which happens to lie outside the hearing range of the small mammals owls typically hunt, allowing the bird to approach its prey without being detected until the final moment of an attack.

The diagram below shows the upper surface of an outstretched owl wing, viewed from above, with the forward border of the wing at the top and the rear border at the bottom.

Label the diagram below. Choose NO MORE THAN TWO WORDS from the passage for each answer.

1. the forward-facing border of the wing that meets the oncoming air first
2. the row of stiff, comb-like points along that forward border
3. the rear border of the wing, from which air finally leaves
4. the soft, irregular texture along that rear border
5. the broad face of the wing lying between the forward and rear borders
6. the fine feather layer coating that broad central face