The other day we got wind of a rare bird occurrence near our Arizona winter home. We went to the nearby artificial wetland and, sure enough, there was a minute red phalarope swimming among the ducks, pecking furiously at minuscule prey near the water surface.

Phalaropes are specialized small shorebirds, unique in that they routinely swim and feed buoyantly on the water instead of wading along shorelines, probing into the mud for prey.

But their differences from typical shorebirds hardly stop there. One of their commonest modes of feeding involves spinning in one spot at about one revolution per second. The movement of the feet while spinning creates a mini-whirlpool that transports small invertebrates upward where they can be snatched by the bill. No other bird species forages in this manner. A flock of these birds spinning at a migration stopover is quite a comical sight! A reliable place and time to witness such flocks is in the saline potholes of central North Dakota in late April.

The social life of nesting phalaropes, however, is what really sets them apart from the pack. This can be summed up in the term “reversed sexual dimorphism.” Simply put, the females are larger and more colorful than the males and take on the usual role of males in courtship, while males assume full responsibility for incubation and brood-rearing. Being freed from incubation, some females may take on a second mate.

Egg-laying, of course, remains the proprietary role of the female in this otherwise topsy-turvy society. Reversed sexual dimorphism is not unique to phalaropes, but is a very rare departure from the normal modus operandi in birds. To date, no one to my knowledge has adequately explained how or why reversed sexual dimorphism has evolved.

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Two of the world’s three phalarope species – the red and the red-necked (not to be confused with the human variety) – nest only at Arctic latitudes across North America and Eurasia.

The Wilson’s phalarope, easily found in May through July in the prairie potholes of North Dakota, nests at much lower latitudes and only in North America. Though mainly a prairie wetland species, in recent decades Wilson’s phalaropes have expanded their breeding range eastward to suitable habitats as far east as Quebec and coastal Massachusetts.

The oddities of phalarope life do not end with the nesting season. Wilson’s phalaropes migrate south through the western half of the U.S. to staging areas on large hypersaline lakes, like Great Salt Lake in Utah and Mono Lake in California. Here, taking advantage of a rich food source of brine shrimp and brine flies, they undergo a rapid and nearly complete molt of the feathers, before fattening up for the rest of the migration. This species is one of only two species of shorebirds known to complete their molt during migration.

To top things off, most of them spend the winter on lakes above 12,000 feet in the high Andes from Peru south!

By contrast, the other two species move quickly from the breeding grounds to the open oceans worldwide, where they spend the remainder of the year before returning to breeding grounds the following year. On the ocean, they favor areas of upwellings that are rich in plankton, like parts of the Bay of Fundy and the Sea of Cortez. The latter is the likely source of the red phalarope we were fortunate enough to see in the deserts of Arizona, following a storm system with strong southerly winds.

This spring, on our road trip back north to Minnesota, we’re hoping we’ll encounter some of those Andes-wintering Wilson’s phalaropes as they spin their way north to the Dakotas and beyond.