Role Reversal: A Closer Look At Wilson's Phalaropes

Across much of the animal kingdom, males are significantly larger than their female counterparts. Further, they assume the responsibility of establishing and defending a territories and are the pursuers in the mate selection process. In many cases they play little if any role in the upbringing of their offspring.

Sexual dimorphism and gender specific behavior patterns also exist in the bird world, with a few twists. Although size differences aren’t usually apparent, in various species, most notably raptors and some shorebirds, females are, on average, larger than males. Additionally, male birds are typically more brightly colored. Through singing and chasing off intruders, they mark and defend the bounds of breeding territories. Finally, male birds typically vie for mating rights, thus it is not unusual to see multiple males pursuing the same female. Even after a pair bond is established, shunned males will occasionally continue to follow the mated pair about for days. A birding friend of mine refers to such threesomes as "a pair and a spare."

Unlike most bird species, the female (right) Wilson's Phalarope is more brightly colored and boldly patterned than the male (left). Females are also up to 25% larger in terms of overall mass. Also note the predominantly brown upper parts of the male, which provide better camouflage when he is on the nest.

One group of shorebirds—the phalaropes—is a rarity among North American birds in that all of the typical rules of size, coloration, mate selection, and parental duties do not apply. Two of the three phalarope species (Red and Red-necked) are nearly circumpolar in their distribution and are strictly Arctic breeders. Across much of North America, Red and Red-necked phalaropes are infrequent to rare mid-continental migrants. Outside the nesting season, they spend most of their time at sea.

Conversely, the range of Wilson’s Phalarope is restricted to the New World. They breed in freshwater wetlands from the Upper Midwest west to the Puget Trough and the southern Oregon coast (limited sites) and from northern Alberta south to the Great Basin. Since Wilson's breeds in areas that are readily accessible to birders and migrates overland, mostly west of the Mississippi River, it is the familiar phalarope for most American and Canadian birders.

In addition to being about 25% smaller (Colwell and Jehl 1994), male Wilson’s Phalaropes have comparatively cryptic plumages, lacking the bright colors and patterns shown by females.  Unlike most birds, phalaropes do not establish a territory before courtship. Females compete with one another for the right to breed with a single male.

Here is another image of the same pair in the photo above. It better accentuates the structural differences between the rather plump and bulky female (right) and the more slender almost dainty-looking male (left). As shown in this picture, male Wilson's Phalaropes usually look smaller-headed and thinner-necked than females.

Once mated, a pair copulates and then creates several potential nest "scrapes," one of which is ultimately chosen by the female shortly before she begins laying (Klima and Jehl 2003). This pair bond is often quite short; pairing and egg-laying are typically completed within 7-10 days (Colwell and Jehl 1994). After clutch completion, the eggs are incubated by the male while the female moves on (often within a day), sometimes to mate with other males. The original male is left to hatch the eggs and tend to the needs of the brood. Young Wilson’s Phalaropes are able to leave the nest almost immediately after hatching. In areas where multiple broods are present, groups of males are known to work cooperatively to protect their offspring from predators (Klima and Jehl 2003).

Here's a closer look at female (above) and male (below) Wilson's Phalaropes.

This curious role reversal has motivated much study of phalaropes, particularly the more accessible Wilson's Phalarope. In his 1997 book “Why Is Sex Fun?: The Evolution of  Human Sexuality” renowned author Jared Diamond offers an insightful chapter entitled “Battle of the Sexes.” In it he discusses the sex-role reversal polyandry of phalaropes and other ground-nesting shorebirds like Spotted Sandpiper and jacanas. He points out that because the young of these species are precocial and can run about immediately after hatching, they are far more self-sufficient than most hatchlings. Thus, they can be cared for by a single parent. The nestlings of most birds remain confined to the nest for many days or even weeks after hatching, necessitating the attention of both parents. 

Though not easily located by the human eye, this Spotted Sandpiper nest, photographed at Fern Ridge Wildlife Management Area, Lane County, Oregon 31 May, would make a quick meal for one of the many skunks, raccoons, and corvids that inhabit this area. This typical four-egg clutch was in shallow grassy cup on the ground just few feet off the trail through the refuge. Predation of such nests is extremely high. These eggs were nearly two inches in length and more than an inch wide.

In order to achieve such advanced development before hatching, the eggs of these shorebird species must be proportionally larger. A typical clutch of four Spotted Sandpiper eggs may account for as much as 80% of the female’s weight (Diamond 1997). Due the effort and energy that is required to produce and lay such large eggs, it is believed that there is an evolutionary advantage to having the male tend to the nest, thereby freeing the female to fatten herself up again and presumably lay more eggs. Diamond (1997) further suggests that this premium on high egg production may be driven by extremely high egg and brood predation among ground-nesting shorebirds.

Despite numerous investigations into the life histories of these birds, there is still much to learn about this type of sex role reversal. Exploring the natural world is seemingly a never-ending process. For every answer there are always two more questions.   

Literature Cited:

Colwell, M. A. and J. R. Jehl, Jr. 1994. Wilson's Phalarope (Phalaropus tricolor), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/083

Diamond, Jared. 1997. Why is Sex Fun?: The Evolution of Human Sexuality. Basic Books, New York, N.Y.

Klima, Joanna and Joseph R. Jehl Jr. 2003. Wilson's Phalarope. Pp. 251-253 in Birds of Oregon: A General Reference. D.B. Marshall, M.G. Hunter, and A.L. Contreras Eds. Oregon State University Press, Corvallis, OR.

All photos taken by Dave Irons