Hi,

I'm no expert on birds, but the first thing that springs to my mind is that there may be a difference in the energy cost of the song flight. Where the more concave wing could work better for this type of flight.

It could also be related to e.g. competition. If they live in a higher density, they may need to fly more often to get noticed (though in this case, the more concave wing would be a coincidence). 

I hope you can find a satisfactory explanation ;)

Anne

A steep ascent as you describe would require a lot of energy. To reduce the aerodynamic cost, you would expect to find an increased wingspan (see attached paper). Is it really that P5-P8 are reduced or could it be that P9-P10 have increased?

The shape of the wing could have something to do with optimising the aerodynamic loading of the wing, but that is a rather complicated problem, depending on many variables....

Article Morphological Adaptations to Song Flight in Passerine Birds:...

Hi,

From my understanding of flight dynamics in birds, I think that, if there is an explainable correlation, it would be related to the energetic costs of vertical flight. Taking off is one of the most expensive moments in flight, costing the bird a lot of energy, which would be greater in a vertical take off.

Shorter primaries, could indicate less drag on the recovery of the wings during the up stroke, for there is less area to create drag. In any case, during the up stroke where the wrist is bent, the primary feathers aligned in such a way that they'll create gaps between them, thus reducing drag. Furthermore, concave structures present more lift than plain ones when pushing down (down stroke).

So, I think you'll need to measure how concave the wings are and relate to wing stroke frequency and body weight and how high the birds fly up, in order to understand if there is any energy cost influence. If so, birds with more concave wings would have an advantage in energy costs and would be able to do more song flights, and thus, getting more noticed (a reproduction advantage).

hope I could help.

Best,

Thank you all for your replays.

We measured only primary feather lengths P2-P8 (and other bimetrics) but unfortunately no wing span or wing area which would be more useful while referring to Hedenstrom and Moller 1992 paper. We used indices of wing shape (C2-pointedness and C3-concavity) as proposed by Lockwood et al. 1998 and we got correlation between towering flight rate and C3 index indicating that birds with more concave wings produces more towers. We also got the negative correlation between towering rate and body mass.

Displaying birds did not ascend as quickly as possible but quite the opposite. To me it looked like they were stalling, and wing-beats were shallow and stiff, without great amplitude or bending the wing typical of quick ascend.

It is generally assumed that more rounded wingtips are beneficial in taking off, but could this difference in wing movements maybe explain our opposite results?

Marco can you explain please what do you mean by that P9-P10 might have increased? I think C3 indicates how rapid is shortening of primaries further away from the wing tip.

Lorian can you please recommend reference where it is stated that concave structures present more lift than plain ones when pushing down (down stroke).

Thank you all again.

Sanja

I have only just seen this, so apologies for the delayed response. When we wrote the wingshape paper we did notice this trend but as best I can recall we did not dig into it, or write about it.

I think the answers you have are all around the likely explanation(s), but I don't see this as a straightforward matter because of just one factor. My instinct would be that either or both the weight or the inertia of the wing are the key factors here. But there are likely to be a whole set of trade-offs going on which will make tying down any single answer very difficult, and might well differ among the birds involved. You don't mention particular species, but I would the examples which come to my mind are larks, with a vertical take-off from the ground or a perch, a climbing flight, and a brief hover in still or near-still air.

In principle the factors which are going to be involved are the best wing designs for take-off, for climbing and/or hovering, for feeding (e.g. insect capture in the air), for manouverability and for sustained flight (e.g. migration).Obviously these apply to different extents in different species/ecotypes. Some of what follows is going to be a bit general, and speculative, and I can't give you any immediate evidence to support all of it....

For vertical take-offs from the ground a bird needs either long legs (e.g. herons and some raptors - not relevant here!) or short wings so that it can achieve a high wingbeat amplitude without hitting the ground, and therefore maximize the amount of air decelerated downwards. BUT short wings can mean reduced wing area, so wings tend to be more rounded, hence more convex/less concave to compensate. Good examples here are galliforms such as grouse, partridge and pheasants, and some corvids (magpies and to some extent jays and choughs). These wings tend to have separated and often emarginated primaries to reduce the aerodynamic drag in the upstroke, and also to reduce wing weight and inertia (to put it crudely because the gaps in the wing tip have no weight!).

For steep climbing flight and hovering the bird has the problem that the upstroke cannot produce much useful aerodynamic force, and it costs muscular work to return the wing to the start of the next powerstroke. The most efficient climbs or hovers would be associated with pressure for a long wingspan, and to reduce weight/inertia the wing will be long and thin, and therefore pointed and more concave. This is probably the dominant factor explaining what you are finding. A wing of this shape is aerodynamically less optimal than the 'average' wing, but that 'average' wing would be heavier and would therefore cost the bird more to move it during the upstroke, requiring more upstroke muscle (supracoracoideus/deltoid), in turn more body mass, and hence (much) more energy to climb and hover. Hence the trade-off.

Migrants would tend to have a long, and 'average' wing as far as possible. Another important - but little considered - constraint in passerines is being able to fly efficiently over long distances with large variation in body mass as fat is consumed. The concave wings would be less optimal here, but again there will be some degree of trade-off.

This last suggest a potential way of looking a bit deeper. Are there differences in song flight behaviour correlating with wingshape between otherwise comparable sedentary and resident species. I know some larks stay in Africa, but don't know whether they maintain the same styles of song flight. Further, are there differences in song flight in migrants through the annual cycle (moult, pre-migratory fattening, etc.)?

Lorian's point about 'concave' wings and lift is strictly correct but not quite relevant to what is being discussed here. It relates to concavity across the chord of the aerofoil section of the wing (and is classical low-speed aerodynamics). The concave index as you know relates to the shape of the wing planform, so is not related. The only way they interact is that a concave wing will have less scope for aerofoil concavity in the wingtip/primary area, and it is the birds with short, rounded wings that have high aerofoil concavity. (With hindsight this is a bit confusion I unwittingly introduced with the term 'concave'!)

Another interesting approach might be to look at differences in take-off dynamics in birds using towering song flights - particularly from the ground or a perch. I am not sure anyone has ever looked at questions like that.

Hope this helps,

Regards

Jeremy Rayner