Maybe females choose the most active male, because he will feed her when she will incubate eggs. And the energy potential of the male can be seen by his songs (sounds) – if louder singing (screaming) male, so he is more active and energetic. Thus, birds can do sexual selection among males  on the song.

I am sure others understand the question fully, but I would love to have it clarified for me.  I can think of three ways in which sex-specific traits might come about througfh selection:(1) because they lead females to choose the males bearing that trait; (2) because males bearing that trait dominate, exclude, etc., other males; and (3) because they favor males in the special conditions of male life not directly related to competition with other males or the attraction of females.  These are not, obviously, mutually exclusive.  Still, it would be nice to know which we have in mind when we speak of "ornaments." 

In great hornbills, where casques are at their pinnacle, one might argue that the ornamentation of the great hornbills are only enticing to females if presented properly by the male.  Thus behavior and the "proper use" of the patterns and ornamentation might be as important as the ornamentation itself. These birds are ritualistic, constantly reaffirming pair-bonds.  Also group courtship has been observed in several species of great hornbill.  So do not discount behavior as a primary selection mechanism.

I imagine you're particularly interested in the evolution of ornaments in great hornbills. Behaviour might therefore not be the best trait to investigate here (unless you can relate it to ornaments). Colour saturation ('yellowness') of the neck/throat and breast feathers and of the wing bar I'd have thought would be of interest. Such presumably carotenoid-based ornamentation might confer a lot of information to a prospective mate about individual and territory quality.

Thank you very much everyone for your responses, they are all very useful indeed. I agree that the question about evolution of ornamentation in hornbills is rather complex. There are a lot of issues to consider, and where or not sexual selection plays a key part or in combination with other drivers. For that matter, degree casque elaboration varies across the family, along with degree of sexual dimorphism, coloration of bare skin, patterns of banding in retrices/remiges, territoriality and practice of cooperative breeding. Even, the use of yellowish-stain preen oil appears to be of some ornamental use. I am considering a way to test models that include degree of song resonance - which may have a direct link to casque elaboration - but still this requires the analysis of all represented song data.

• Still interested in how you understand "ornament" in relation to the three selection "pressures mentioned above.  Or is it "none-of-the-above"?  Is it, a "structure that doesn't seem very useful in everyday life?" 

Thank you Nicholas for your response and keen interest in my study of ornamentation in hornbills, much obliged indeed. My apologies if my query was rather unclear. Previous hypothesis on casque evolution had centered on the use of the casque as support for long bill length, or for improving resonance of calls, as well as possible heat regulation. Others suggested that casque elaboration was a result of sexual selection, since males (in highly dimorphic taxa) have more elaborate casques. In some hornbills, the casque is less pronounced between sexes, but show differences in plumage coloration or bare skin color. I have tested models for casque evolution based on body mass, territoriality and diet, and now interested in testing for effects of sexual selection, if any. It appears, I need to look into relationship of degree of dimorphism based plumage/skin coloration alongside casque elaboration. I am unsure if these traits are enough to sufficiently understand ornamentation in hornbills. However, I am fishing out suggestions on how I best test all possible drivers for ornamentation in this charismatic group of tropical birds.

Is the casque just a casque functioning as nothing more than a casque? is how I understand your question.

My answer to that would be that we do not necessarily understand enough the function of the casque and for such an elaborate "ornament" to evolve probably involved some fancy ecological pressures to interact. The use of casques for different purposes has not been shown except for ageing and butting. Simply ageing does not make sense since casques develop into their final forms only later in a bird's life. Hence, although sexual selection is the most commonly cited reason, I suspect that there are more as hitherto undiscovered uses which will astound us as to its uses. Complex vocal communication comes immediately to mind and should be interesting to check for the spectrum of their vocalizations - are they producing anything  beyond our capabilities? - is only one idea.

Good luck with your study

I tried to send you a side-noet off line  just to let you know I am still with you, here, but I couldn't turn up your address. 

Yesterday, I took an hour to write what I have to confess, in complete modesty, was an astoundingly wise contribution to this thread but then made the mistake of shifting webpages to check a reference, and lost it.  The biggest fish is always the one that gets away, no?  I will try to reproduce it later, but right now I am in deep mourning and all my shades are drawn.  Please keep this interesting thread going. 

If you do a plain-old Google search on "casque", there is an extraordinarily large set of illustrations of casques of all sorts.  One striking feature is that the INTERNAL structure seems to vary a lot, as if casques were being made out of somewhat different "materials", so to speak. Are we talking convergent evolution here?. Conversely, do we know anything about the embryolodgy of casques?    Are we perhaps in some sort of territory such as the elaborate pseudopenis of the spotted Hyena?  Think "spandrel". 

Great thread you have going, here.  A tribute to Niko Tinbergen. 

N

Thank you very much indeed for your invaluable responses. I agree that looking into the complexity of vocalizations in relation to casque elaboration would be an important part of this study, as with the development biology of the casque itself and variation in the internal structure. The solid casques used for aerial jousting by Helmeted Hornbills are definitely unique, although to some degree head-butting behaviour has been observed in other species with large hollow casques (possibly more of casque-tapping), so there is some degree of evolutionary relations that led to jousting. Given this, how did these materials develop to reinforce the skull for this extreme confrontation?