Why Haven't Birds Evolved Tail-fins?

[Dekan]

When birds fly with their wings flapping, they go in a smooth straight line.

 

But when they stop flapping, and just glide through the air, the glide seems quite "wobbly". The birds keep tilting and swerving slightly, and give the impression of directional instability.

 

This instability results from lack of a vertical stabilizer, ie, a tail-fin.

 

Tail-fins are fitted by humans to aircraft. And fish have evolved tail-fins. Surely birds could easily evolve a vertical fin. It could be just a sort of stiff planar ruff of feathers on the rear of their bodies.

 

Such a feather-fin would "cost" very little - it would add hardly any weight, and require no radical change to the avian muscular/skeletal structure.

 

So I wonder why evolution has so far failed to equip birds with one. Are there any signs of one evolving in modern birds?

[Cap'n Refsmmat]

It seems to me that this would afford very little additional stability for the conditions a bird flies under. Most birds have relatively short bodies, meaning a tail fin would have to be very large -- you want control surfaces to be as far away from the center of gravity as possible to make them effective. If the fin has no additional muscular structure to allow it to move under the bird's control, it would improve stability at the cost of decreased maneuverability as well.

 

In evolution, however, the big question is "does this lack of yaw stability constitute a disadvantage for survival?" I think birds are able to control their yaw through their existing tail feathers and wings, so perhaps the answer is "no," and so there's no evolutionary reason for a vertical tail to become widespread.

[Ophiolite]

But when they stop flapping, and just glide through the air, the glide seems quite "wobbly". The birds keep tilting and swerving slightly, and give the impression of directional instability.

I think you are misinterpreting what you see. Flight patterns of birds vary, but consider for a moment seagulls. As they glide alongside a ship they can maintain a very level flight with considerable stability. When they do 'tilt and swerve' it is for one of two reasons: to better position themselves to look for food; to adjust to minor air currents. Similar behaviour can be seen on the parts of hawks and kestrels.

 

Fine control of feather positions affords all birds an excellent aerial stability. Evolution has served them well. A large 'tailplane' would be a serious disdavatange and poorer solution to what they have in place.

[Edtharan]

Unlike planes, birds have a very sophisticated feedback mechanism to control their stability. There have been some planes that use unstable flight characteristics and a feedback system for stability control to increase their manoeuvrability.

 

I think this would work for birds too. So having an unstable flight characteristic with a feedback mechanism would allow birds to be more manoeuvrable. This would aid them in avoiding predators, or to target prey themselves. Also, flying in confined spaces would be easier too with such a system in place.

 

So, as Cap'n Refsmmat said, there is no real advantage for birds to develop tail fins. And, there could actually be a disadvantage to developing them.

[Moontanman]

I think it's reasonable to say that birds did at one time have a tail fin, they had long tails that dangled down to provide stability, it didn't stick up like a airplane but it served the same purpose and it was lost in the evolutionary struggle at the K/T extinction and only birds with the rudimentary tails they have today made it through that bottle neck, the birds that all modern birds descended from lost their tails, possibly due to better brains, so they no longer needed them?

[Dekan]

Thanks to everyone for your kind replies. These have provided the answer:

 

Birds don't need a a vertical tail-fin, because they have an on-board computer (the avian brain). This brain rapidly actuates horizontal control surfaces, and so confers stability without the fin.

 

In the same way that the computer brain on a B-2 bomber confers stability, even though the B-2 hasn't got a fin.

 

(The B-2 pilots' brains don't do anything except decide whether that's the right target underneath, then pull the bomb-release lever).

 

Sorted! Thanks again.

[Hal.]

Edit : I had an opinion and question but I now see my point of view has been considered already .

Edited June 8, 2011 by Hal.

[npbreakthrough]

evolution doesn't give animals what they need..........