Countertenors are popping up like once-rare flowers. At New York City Opera, they've become a virtual fixture during the past decade, what with star turns by the likes of Bejun Mehta in Handel's Partenope, Ariodante, Flavio, and Orlando; David Daniels in Xerxes and Rinaldo; and Matthew White in Orlando and Semele. This month's revival of Handel's Flavio boasts two choice representatives of the species: longtime favorite David Walker, whose supple sound City Opera audiences have admired in Handel's Partenope, Agrippina, and Xerxes, and rising star Gerald Thompson, who is making making his company debut.

Countertenors have been on the rise for decades. The recent flurry of interest in Handel's operas, spearheaded in this country by City Opera, is one reason why. True, the company's pioneering 1966 production of Giulio Cesare starred the great bass Norman Treigle in the title role, normally undertaken these days by a countertenor or a mezzo-soprano. Wait — A countertenor or a mezzo? Oh, dear: gender ambiguity rears its bespangled head.

Most of the Baroque operatic roles now claimed by countertenors and mezzos were designed for a now-extinct voice type, the castrato. Yes, for centuries prepubescent boys were surgically altered so that the high, liquid tones of their youth might remain intact. The practice was first documented in the 1500s but probably had begun centuries before, in response to rules prohibiting women from singing in church. The 1994 film Farinelli evokes the rock-star glamour and wealth that leading castratos enjoyed. Contemporary accounts — along with the music written for castratos — tell of artists possessed of mind-boggling breath control, huge vocal ranges, and dazzling virtuosity, along with the high-flying notes of female singers; Farinelli used an electronic blend of the voices of a countertenor and a female soprano to approximate this sound.

In the Baroque era, which still entertained the notions of demi-gods and rulers by divine right, castratos portrayed leaders both benevolent and depraved, and characters with supernatural (i.e., "more-than-natural") powers. City Opera audiences are familiar with such exemplars as Handel's Orlando, the paladin whose amorous madness rends nature and human society; Monteverdi's Nero (L'incoronazione di Poppea), who suffers no limits, erotic or otherwise; and Gluck's Orfeo, the bard whose song tames savage beasts and death itself.

So who are these countertenors who have inherited the onerous mantle of the castratos? Literally, the word means "against the tenor," since the countertenor in the multi-part music of roughly 700 years ago sang a vocal line that riffed on the tenor's. As critic J. B. Steane notes, "Unlike terms such as 'alto' and 'bass', ['countertenor'] does not describe the voice but, like 'tenor' itself, indicates a function." (The tenor, wayward and temperamental according to today's stereotypes, was a model musical citizen back in the day, singing the grounding melody upon which polyphonic fantasy was spun.)

A function and not a particular type of voice: The countertenor made his entry onto the musical scene without a fixed identity of his own. Controversy rages as to what countertenors sounded like in centuries past: They may have been high tenors or similar to altos; and they may have used falsetto — which may or may not be equivalent to "head voice" (ethereal tones produced without the beefy resonance of the chest).

After slipping into oblivion for hundreds of years, countertenors resurfaced some 60 years ago in the first days of the early-music revival. Alfred Deller, a former alto in the choir of England's Canterbury Cathedral, championed the music of Henry Purcell and called himself a "countertenor" in honor of the great 17th-century composer, who had himself sung that part. Deller's caressing, luminous sound also inspired contemporary composers to write for him — for example, Benjamin Britten wrote the role of Oberon in A Midsummer Night's Dream for Deller.

David Walker, who sings the title role in Flavio at NYCO this month, remarks on how the voice type has evolved in recent decades. Deller and Russell Oberlin — another distinguished mid-20th-century pioneer — blazed "a great path for modern-day countertenors," says Walker, but their training did not lend itself to the challenges of Handel's great operatic roles. At the same time, with modest self-awareness, the NYCO star adds, "Much as modern countertenors want to believe that they are sounding like castratos, I don't believe we sound as spectacular as they did." He ticks off a list of vocal qualities — "a more natural sound, ease of vibrato, evenness of range, correctness of vocal 'ring' in the voice to carry as well as you can over an orchestra" — that form the basis of all modern-day operatic vocalism, but that were not necessarily cultivated in the church choirs and early-music ensembles of a half-century ago.

Indeed, pressed about the particular difficulties of Flavio, Walker emphasizes not the role's sweeping vocal requirements — ranging from the ability to spin a light, tripping line to the agility to handle Handel's florid writing — but the dramatic challenge that Flavio presents. "It is difficult to pull off a spontaneous, silly, and love-struck king and still make him believable as a king — while winning audience sympathy for his character! Plus, NYCO's production of Flavio is extremely demanding in terms of physicality and comic timing. I have to 'ride' the character right up to the line, but I can't cross over it!" Still, Walker acknowledges, "It is one of my more cherished performing experiences."

While some modernday countertenors have struggled to come to terms with their unusual sound — some personally, some vis-à-vis unsupportive teachers — Walker, a latecomer to operatic training, quickly realized he was comfortable singing in a higher range and found a mentor who encouraged his vocal explorations. "I had only heard countertenors in Anglican choral music or lute songs, which are beautiful, but I was beginning to appreciate opera and wanted to sing more of that kind of music. Within a few months, [my teacher and I] both realized that a great variety of music was available to me."

For all his humility, Walker does share with his castrato semi-forbears a love for glamour — and a touch of gender-bending daring. He counts among his vocal idols Freddy Mercury of Queen and Robert Plant of Led Zeppelin, describing Queen's late leader, dulcet-toned and "out and proud," with a salty epithet that suggests gonzo vocal and sexual prowess — stereotypes about "effeminate" men be damned. And he confesses, "I also do a very good imitation of k.d. lang and love to make up harmonies around her voice. A secret dream is to sing with her one day!"

Today, comfort with gender-bending sounds has evolved to the point that a high-voiced male pop singer like Antony Hegarty of Antony and the Johnsons receives the serious attention he deserves — instead of being dismissed as a circus act like the ukulele-playing Tiny Tim. No longer the odd man out, modern countertenors are embraced by both composers and impresarios. Both Philip Glass and Peter Eötvös, among others, have written substantial new roles for them (in Akhnaten and The Three Sisters, respectively), and countertenors now boldly claim music once considered off-limits to them, including the role of Cherubino in Mozart's Le nozze di Figaro (written for a woman) and Berlioz's song cycle "Les Nuits d'été."

Scholars continue to debate whether music written for castratos is better sung by, say, a modern countertenor or a mezzo-soprano. But when the choice is between arias for Farinelli sung by, say, Vivica Genaux or David Walker, how can audiences lose?

Marion Lignana Rosenberg blogs at www.vilainefille.com and writes frequently about the arts.