I've long been fascinated with arms and have recently begun educating myself in earnest, with a goal of becoming knowledgeable enough to purchase some historical examples confidently in due time. However, as with anything, the more you learn, the more questions you have! :D I've included my questions below, italicized.

I have come to wonder: why does the European halberd change shape, from a fairly simple two-eyed axe (with hook/beak and spike, of course), first towards a forward-canted straight edge and then towards a concave crescent shape?

I recently read Dr. Snook's article here in which he discusses the change from straight bit to a forward cant (or, as he calls it, "oblique") angle straight bit, which he ascribes to the desire to slash or slice more than to chop. First: why was this desire present?

He goes on to discuss the change thereafter towards a crescent shape. Although he notes the evolution of the weapon toward the crescent shape, and notes the reinforced profile these points were given, he doesn't explain why it happened. Am I wrong to assume this change is due to the proliferation of armor? Is there another explanation?

Another question: why do so many historical examples show holes in the axe bit and/or beak? Are they purely decorative, or was there some other cause?

(I have a gallery of images I can link if anyone is unclear as to what I mean, but I list the source of the images there as an auction site, because disclosing sources is important, and I don't want to run afoul of the rules of this lovely board without being certain that linking to an imgur gallery of halberd photos is alright even if they make mention of an auction site to whom I have no relation, professional or otherwise.)

Thank you kindly for your expertise on this subject, ladies and gentlemen!

Well, holes and cutouts do also make the blade lighter.

Let's also not forget that a lighter blade on a longer pole can hit just as hard as a heavier blade on a shorter pole, so it's not at all necessarily a trade-off on impact energy. I think a more important factor than armored targets becoming less common would be that the prevalent battlefield conditions and tactics simply turned in the favor of longer weapons (although a part of that would be the halberdier himself perhaps wearing less armor, making a little more distance from the enemy a thing to be desired).

I must admit I had not researched deeply when I presumed the smaller heads were lighter. It is true that they may be thicker despite appearing shorter, and retain similar weight properties. I'll have to defer to the experts on that one! :)

Timo Nieminen wrote:

Waldman, "Hafted Weapons ...", http://myArmoury.com/books/item.9004144099.html describes the evolution, including the trend towards lighter heads. (Danny F., this is a great book.)

That looks absolutely fantastic! Thank you for this recommendation.

Regards to all, and thank you everyone for the discourse here on this topic. It has been very, very helpful and informative.

Bear in mind that in latter years, especially towards the mid or late 17th century, halberds became far more of a decorative parade and guard item than an actual weapon. That also plays a part. When weapons start sliding towards obsolescence, they tend to become exaggerated and more ornate as a response to their increased lack of utility. Note the flashiness of the cinqueada versus the simplicity of knightly swords.

Of course, this also goes hand-in-hand with changing fashions at the time; the cinqueada was a Renaissance period sword, and the 17th century was the Baroque period (for the most part) in art, so you see concurrent increases in the decorative arts.