Wilson's Phalarope

Wilson's Phalarope operates a polyandrous mating system in which sex roles are reversed compared to most birds. Females arrive on the breeding grounds before males and actively compete for mates, sometimes engaging in physical fights with rival females. A single female may mate with multiple males in one season, laying a clutch for each and leaving the male to handle all incubation and chick-rearing while she seeks the next partner.

Courtship is initiated by the female. She pursues males with aggressive postures, whipping her head back and forth, and uses bowing and upright displays to initiate copulation. Aerial chases and water-surface displays are also part of the repertoire, with the female swimming towards the male with neck held erect to show off her bold plumage. DNA studies have found no evidence of extra-pair fertilisations — despite the polyandrous system, each clutch belongs to a single pair.

The female selects the nest site, typically on the ground near water, sometimes slightly elevated in marsh vegetation. The male then tidies the scrape and arranges surrounding grasses or sedges to conceal it. The nest is a shallow depression lined with grass. Clutch size is almost invariably four eggs, laid at approximately 24-hour intervals between early May and late June. Eggs are buff-coloured, blotched and spotted with brown and black, measuring approximately 3.2–3.5 cm long by 2.3–2.4 cm wide.

Incubation is performed by the male alone and lasts 18–27 days, typically around 23 days. Chicks hatch with eyes open and are fully feathered and mobile within one day. They are precocial, finding all their own food from the start. The male broods the chicks while they are small and performs a broken-wing distraction display if a predator approaches. Young are capable of flight at approximately 19 days of age — an unusually rapid development. The species can produce one or two broods per season, and young reach breeding age at one year old.

Wilson's Phalarope typically lives between 5 and 10 years in the wild, with a maximum recorded age of around 10 years. This is broadly comparable to other small shorebirds of similar size, such as the Dunlin and Wood Sandpiper, though the long-distance migratory lifestyle imposes significant annual mortality risks.

The most dangerous periods of the annual cycle are the migration itself — particularly the long overland and transoceanic flights between North and South America — and the staging period on hypersaline lakes, where the entire population is concentrated and vulnerable to sudden environmental changes. The near-total desiccation of Lake Abert in 2021–22 and the record-low water levels at the Great Salt Lake in 2022 illustrate how rapidly conditions at these critical sites can deteriorate.

Predation on the breeding grounds is managed partly by the male's broken-wing distraction display and partly by the cryptic colouring of the nest and eggs. Known predators include birds of prey, foxes, and mustelids. Young birds, which fledge at just 19 days old, face the highest mortality rates — as in most shorebirds, first-year survival is substantially lower than adult survival. Birds that survive to adulthood and successfully navigate the annual migration cycle can reach the upper end of the typical lifespan range.

Wilson's Phalarope is currently listed as Least Concern on the IUCN Red List, with a global population estimated at 1–1.5 million mature individuals. However, this status is increasingly contested. International Shorebird Survey data indicate a population decline of approximately 70% since the 1980s — a trajectory that prompted a formal petition in March 2024, led by the Center for Biological Diversity, to list the species as Threatened under the US Endangered Species Act. The species is already listed as Threatened in Minnesota and is a US Fish and Wildlife Service Bird of Conservation Concern.

The most acute threat is the desiccation of the three hypersaline staging lakes on which up to 90% of the world's adult population depends simultaneously. The Great Salt Lake — used by up to 60% of the global population — fell to its lowest recorded level in 2022, temporarily becoming too saline for brine shrimp reproduction. Lake Abert in Oregon has nearly dried up twice in recent years (2014–15 and 2021–22). Mono Lake in California remains chronically below its legally mandated minimum water level. The collapse of any one of these lakes would be catastrophic for the species.

Drainage and conversion of prairie wetland breeding habitat — primarily for agriculture — continues to reduce available nesting sites across the Great Plains. Water diversion for irrigation reduces inflows to saline staging lakes. Climate change is exacerbating drought conditions across the western United States, accelerating the decline of these critical water bodies. On the wintering grounds in the Andes, lithium mining operations pose an emerging pollution threat to the saline lakes that support wintering populations.

The Migratory Bird Treaty Act provides some legal protection, but conservation advocates argue that without formal Endangered Species Act listing, the pace of habitat loss will outstrip the species' ability to recover. The contradiction between the IUCN's 'Least Concern' designation and the documented 70% decline reflects a lag in the assessment process rather than genuine stability.

The courtship system of Wilson's Phalarope is one of the most thoroughly reversed in the bird world. Females arrive on the breeding grounds before males and immediately begin competing for mates. A female will pursue a male with aggressive postures — whipping her head back and forth, puffing out her neck feathers, and occasionally engaging in physical fights with rival females. The female's bold black neck stripe and salmon-pink foreneck are not merely decorative: they are signals of competitive quality directed at both rival females and prospective mates.

Once a female has secured a male's attention, she uses bowing and upright postures to initiate copulation. Aerial chases are common, with the female pursuing the male in low, rapid flights over the marsh. On the water, she swims towards the male with neck held erect, displaying her plumage and uttering repeated low calls. The male's role during this phase is largely passive — he is the choosy sex, assessing female quality before committing to a pair bond.

After mating, the female selects the nest site and the male tidies and lines the scrape. Once the clutch of four eggs is complete, the female typically departs within days to seek a new mate, leaving the male as the sole incubator. A productive female may secure two or even three mates in a single season. DNA analysis has confirmed that despite this system, there is no evidence of extra-pair fertilisations — each clutch is the genetic offspring of the pair that formed it, suggesting that females invest their competitive energy in securing exclusive access to high-quality males rather than in promiscuous mating.

Wilson's Phalarope undertakes what biologists call a 'molt migration' — a phenomenon documented in only two shorebird species worldwide. Rather than moulting on the breeding grounds or the wintering grounds, birds travel to specific intermediate staging sites where they simultaneously undergo a complete body moult and build the fat reserves needed for the final leg of their journey to South America. The staging period lasts approximately 35–40 days, during which birds replace all body feathers — one of the fastest complete body moults recorded in any bird.

The three critical staging sites are the Great Salt Lake in Utah, Lake Abert in Oregon, and Mono Lake in California. These hypersaline lakes support extraordinary densities of brine shrimp (Artemia spp.) and brine flies (Ephydra spp.) — invertebrates that thrive in water too salty for fish, eliminating predation pressure and allowing prey populations to reach astronomical numbers. Up to 90% of the world's adult Wilson's Phalaropes can be present at these three lakes simultaneously between late June and early September.

The southward migration is staggered by sex and age. Females depart the breeding grounds first — as early as late June, immediately after laying — followed by males once the chicks are independent, and finally juveniles in late summer. At the staging lakes, birds gorge so intensively that they can more than double their body weight, accumulating the fat reserves needed for non-stop flights of over 54 hours to reach South America. The vulnerability of this system is stark: the near-total desiccation of Lake Abert in 2021–22 and the record-low Great Salt Lake levels of 2022 temporarily eliminated the food supply at two of the three sites, with potentially severe consequences for the global population.