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When ace Montana birder Radd Icenoggle spotted an odd duck south of Missoula in April 2013, it didn’t take him long to identify it: a male . This Asian dabbler, a very rare visitor to North America, was never sighted in Montana before, but its sweeping back feathers and dazzling face pattern made it unmistakable.

The Baikal Teal stayed around for several days and was enjoyed by many birders, but arguments flared: Was this a wild bird, or did it escape from captivity?

Such questions about waterfowl have plagued birders for years. There are about 160 species of ducks, geese, and swans in the world, and most kinds are kept in captivity in either zoos, aviaries, or private waterfowl collections. These valuable exotics are usually held in safe enclosures, or wing-clipped to make sure they don’t stray. But through accidents or neglect, some of the birds may break out, wandering in the wild for weeks, or even years, periodically popping up at popular birding spots. The escapees may be beautiful to look at, but they don’t count on life or state lists. Thus, they can be vexing to birders.

Most states and provinces now have expert committees that pass judgment on rare-bird reports. For your own list, of course, you don’t have to follow the dictates of committees. But how would you decide whether a particular bird was wild or not? Here are the facts you should consider while answering the riddle.  

Geographic logic. Some species of waterfowl are unlikely to reach North America on their own. Black Swans, for example, are native to Australia and introduced in New Zealand. There’s no reason to believe they would ever fly all the way across the Pacific, so when one shows up on a pond in New Jersey, we assume it’s a local escapee.

Other cases aren’t so clear-cut. For example, the White-cheeked Pintail is common in the Caribbean, including throughout the Bahamas and within 70 miles of the Florida coast. When one turns up in Florida, there’s a good chance it’s a wild bird. But what about sightings farther up the coast, in North Carolina or Virginia? Still reasonable, perhaps. Texas? California? Now you’re pushing it.

Migratory habits. The Hottentot Teal, native to Africa, only moves short distances. When it’s seen free-flying in North America, it’s assumed to be escaped from captivity. On the other hand, another type of teal, the Garganey, migrates all over the Old World, nesting across northern Europe and Asia and wintering as far south as Africa, Borneo, and New Guinea. Long-distance migrants are much more likely to stray outside their normal range, and Garganeys are the perfect example. They’ve shown up dozens of times in North America, with scattered records all over the U.S. and Canada (one was confirmed in upstate New York ).

Good timing. This is usually just a minor clue, but if a rare duck or goose shows up at a time that fits with normal timing of migration for the species, it helps to indicate that it’s a wild bird. A Common Shelduck on the lake in midsummer probably doesn’t belong in nature—or on your list.

Status in the wild. Knowing the population trends of waterfowl can be helpful. Consider Barnacle Geese and Pink-footed Geese in northeastern North America, which both nest in Greenland and Iceland, migrating across the North Atlantic in fall to spend the winter in northwestern Europe. A few decades back, most Barnacle Geese and the few Pink-footed Geese spotted in North America were thought to be escapees. Now, both species are being found frequently in eastern Canada and the northeastern United States, and most are thought to be genuine wild strays. Why the change in opinion? It’s mostly because both of these geese have seen a big hike in their wild populations in recent years. With increasing numbers, there’s increasing potential for a few birds to go off course.

Status in captivity. Some exotic waterfowl are more popular than others. For instance, Red-crested Pochard and Mandarin Duck are common in U.S. zoos and private collections. On the other hand, the Masked Duck is rarely, if ever, kept in captivity. In some cases, you can research whether a species is captive by consulting , a collection of databases from zoos, aviaries, and aquaria worldwide. (For many years this group was called International Species Information System, but recently the acronym, ISIS, became too much of a liability.) It can be tough to prove that a species is NOT kept in captivity, because Species360 doesn’t track all private collections. But it’s a start.

Signs of captivity. Many keepers of exotic waterfowl mark their birds by clipping the hind toe (hallux) on one foot, or putting a colored band on one leg. Birders who discover these questionable birds may spend hours waiting for their subject to get out of the water onto dry land so they can see its feet.

Now, with all these clues in mind, let’s go back to that Baikal Teal in Montana. Here are some specifics from the duck’s file.

• The species is widespread in eastern Asia, migrating from northern Siberia as far south as southern China, so it’s a long-distance migrant and not half a world away. It has been found rarely in Alaska and very rarely in the western United States and Canada.
• April would be a reasonable time for a vagrant to be migrating north.
• As recently as the 1980s, Baikal Teal were considered rare and vulnerable in their native range, but numbers wintering in South Korea have greatly increased in recent years, so the species seems to be doing well.
• The teal is not commonly kept in zoos, and the bird seen near Missoula showed no signs of having been a captive.

The verdict? The decided this was a wild bird, and added Baikal Teal to the official state bird list. But the fact that it took a handful of experts to reach that conclusion shows just how twisted these riddles can be.