We visit the zoo so we can experience wild animals in captivity, but the side benefit is that we also get to enjoy the architecture. Since these menageries are fantasy environments, zoo buildings are liberated from their usual social obligations. They can be almost anything they want to be.

This was evident from the very first public zoos, which began appearing in Europe in the mid-19th century. They were populated by a cast of delightful follies - from Hindu temples that sheltered lumbering elephants to colorful Moorish castles where African giraffes grazed among the minarets. The more exotic, the better.

By the time America's first zoo opened in Philadelphia in 1874, keepers were more interested in science than sideshows.

The Philadelphia Zoo sought out Frank Furness and George Hewitt, architects known for grafting the Victorian era's picturesque sensibility onto an emerging industrial aesthetic, one that celebrated technological innovation and the toughness of new products such as steel. As zoos evolved, the scientific side came to dominate. Over time, zoos ditched their buildings and cages in favor of environments that replicated the animals' wild habitats.

The McNeil Avian Center, which opens to the public May 30, is the latest effort by the Philadelphia Zoo to provide its charges with a naturalistic setting. The zoo's impressive collection of tropical birds can now fly free in a spacious, sun-filled enclosure that mimics their native environments.

Actually, in terms of area, the new fly zone isn't all that much bigger than what existed in the old bird house. But because the ceilings now are significantly higher, and because the exhibits are so cleverly designed, the $17.5 million space feels more expansive. If only the whimsical spirit of the exhibit designers could have included the exterior of the addition, which errs on the side of bland functionality.

The renovation and expansion of the zoo's old bird house, designed in 1916 by the venerable Philadelphia team of Mellor and Meigs, was overseen by the architects at SaylorGregg, formerly DagitSaylor.

In such projects, the architects generally do most of the heavy lifting: They provide the building envelope and operating systems, while exhibit designers get to fill the place with all kinds of fun stuff. The new bird house boasts evocative habitats, bright signage, and a charming introductory film - created by, respectively, the Portico Group, Main Street Design, and Science North.

It made a lot of sense for SaylorGregg to retain the old bird house. Though the neoclassical facade is as plain as they come, the compact little aviary occupies what is probably the most picturesque spot in the zoo, overlooking a small pond dotted with swan boats.

In the late '60s, the zoo slapped a boxy addition on the back. The roof was kept low - about 10 feet high - probably because the old bird house itself isn't very tall. As an early effort at creating a free-flying habitat for its birds, it was admirable. But for flightless, two-legged visitors, the cramped space wasn't particularly pleasant. SaylorGregg's challenge was to make space soar without overwhelming the little neoclassical temple.

When viewed from the pond, it's clear they've done a fine job of camouflaging the addition; you hardly know the new roof angles up to 30 feet. Once you step inside, through the new side-door entrance, you're immediately drenched in a lovely dappled light - not to mention some seriously tropical air, controlled at 85 percent humidity. You come face-to-face with the aviary's prize couple, a gleaming pair of black rhinoceros hornbills from Indonesia, endowed with an impressive double-stack of yellow-orange beak.

Besides being bright and airy, the interior of the SaylorGregg addition is immediately legible. As soon as visitors enter, they know intuitively where to go next. Turn right to begin the progression through the exhibits, which are separated by barely visible wire curtains. Turn left to access a 45-seat theater.

My gripe is entirely with the exterior, but it's an important one. The large new addition has a high-profile face of its own, overlooking the sloping grasslands of the African Plains exhibit, where pairs of addax - a type of antelope - graze. The new facade serves as backdrop to this pastoral scene, and it should have been a strong architectural presence.

Instead, it's about as exciting as a service building. You half expect the boilers and chillers to be housed there.

True, SaylorGregg had to deal with some complications. Since the zoo didn't want its birds perching on a steel framework inside the exhibit area, SaylorGregg was obliged to locate the support structure on the outside of the building.

Rather than constraining the architecture, that requirement offered SaylorGregg an opportunity to celebrate the building's bones - in biological terms, its exoskeleton.

But there's no verve or grace in the unpainted steel grid, which lacks the detailing and joint-making craft that might have infused personality into such a functional system. Even worse, the dull steel pipes are layered against a milk-colored wall made largely of acrylic panels, to let light shine on the exhibits. The slightest splash of paint would have been an improvement.

As Furness and Hewitt realized more than a century ago, a steel frame can be beautiful thing. But only if the architects and the client insist on making it that way.

Contact architecture critic Inga Saffron at 215-854-2213 or isaffron@phillynews.com.