This design is usually referred to as ‘fingers’, ‘fingertips’ or ‘spread fingered’ and describes the separated feathers (outer primaries) at the end of the wing in flight. Found in large, heavy birds that have large, long, broad wings and that use these wings for soaring.
The movement of a wing through the air causes turbulence as the air current is divided to go over and under the wing. The air travelling over the top of the wing travels faster than the air under the wing causing different air pressures on the surfaces. The pressures need to balance out and they attempt to do this at the edge of the wing, causing a lot of vibration that makes the wing shake and the bird then battles to fly. To prevent this vibration, the feathers at the end of the wing have gaps (slotting) to allow the air currents to mix and reduce the vibration, allowing the bird to maintain stable flight. Smaller birds have less vibration because their wings are smaller and therefore do not need slotting.
What are the reasons for different tail shapes?
As with wing shape, tail shape is inadaptation to habitat and the type of food eaten and technique required to get it. Although the power and lift for flight is generally all developed by the wings, the tail is vital for steering the bird and enabling various degrees of manoeuvrability. Generally, the longer and bigger the tail, the more stable the bird will be in flight. This stability comes at the expense of turning ability, because it stands to reason that the more unstable something is in flight, the easier it is to deviate from the flight-path. The opposite is, of course, also true.
Birds that catch food in the air – such as swallows, swifts and pratincoles – generally have forked tails with a small surface area. These birds need to be agile and manoeuvrable to catch their prey. The forked tail also has the advantage that it can be spread or fanned to change the shape (and therefore the surface area) and thus the turning ability at different speeds – with fantail being slower and streamlined tail being faster. Small, unstable tails are taken to extremes in the spinetails where only the bottom parts of the feather shaft have a vane, resulting in spine-like protrusions – hence the name. Another indication of the inherent instability of short tails is to look at the bateleur. Sometimes incorrectly referred to as the short-tailed eagle, this raptor has to compensate for instability in flight by continually rocking from side to side with the wings open while gliding and soaring. This is, in fact, how it got its name, meaning tightrope walker in French.
Larger birds of prey and vultures, for example, will also fan their tails when soaring to increase the surface area for lift. Broadening the tails will also assist with breaking during aerial manoeuvres (such as in bee-eaters and sparrowhawks) and when the birds need to decelerate in order to land. Some birds have tails that get longer towards the centre with very firm shafts, allowing the tail to be used as a support structure/prop when clambering around on vertical surfaces (e.g.: woodpeckers and oxpeckers). In many other species, the tail design is also influenced strongly by social pressures. The males of many, in particular, will develop long streamers when breeding in order to show off – the change often going hand in hand with striking plumage changes (e.g.: widowbirds, paradise flycatchers and whydahs). Because these tails hinder flight and make the bird vulnerable and more conspicuous, they can only be tolerated seasonally. The stiffened feathers and design of the tail may also be used in display flights to make sounds in breeding displays (e.g.: African snipe and some larks). Long, rectangular tails also come in handy in raptors such as sparrowhawks, which used the tail to brake and steer through the forest understory.
Excerpts from ‘Beat about the Bush – Mammals and Birds’ by Trevor Carnaby, Published by Jacana Media, Second Edition reprint 2018.
