Editor’s Note: Entry number eight in our tenth anniversary retrospective is twelfth in popularity, and is a three-part series, originally run July 19-21, 2016. Links to parts two and three are at the end. My interest in the subject was sparked by seeing the polyandrous Eurasian Dotterels on their breeding grounds in northern Norway, and the ensuing discussion.  [Chuck Almdale]

Sometimes it’s the exception that proves the rule.

Female Wilson’s Phalarope, Malibu Lagoon, Ca.
(Grace Murayama, June 6, 2016)

Beginning birders soon notice that in many species, the male and female look different from each other. This divergence in appearance linked to sex appears in both birds and mammals. In mammals, for example, male lions have large manes that females totally lack, gorilla males are larger than females, male deer annually grow and shed their large antlers, and, of course, human males and females differ in body size and shape.

Irish Elk Group (Tabitha Paterson, TwilightBeasts)
Irish Elk size comparison (prehistoric-wildlife.com)

This distinct difference in features between genders is commonly called sexual dimorphism, or sometimes sexual bimorphism (two forms). The single-form alternative is sexual monomorphism (one form).

Sexual monomorphism in Snowy Egrets, Malibu Lagoon, Ca.
(Jim Kenney,  November 2006)

Some examples of sexual monomorphism commonly seen in California are: geese, swans, loons, grebes, cormorants, pelicans, most herons and egrets, vultures, most sandpipers, gulls, terns, alcids, typical owls, swifts, corvids, chickadees, thrashers, some warblers, and most sparrows.

Common Murre (bridled variety), an alcid, exhibit sexual monomorphism.
Hornoya, Norway. (Joyce Waterman, May 30, 2016)

In California, readily seen examples of sexual dimorphism are: ducks, quail, grouse, phalaropes, hummingbirds, kingfisher, woodpeckers, gnatcatchers, most warblers, tanagers, blackbirds and orioles, finches, House Sparrow.

Hooded Mergansers exhibit sexual dimorphism; male front, female behind
(Jim Kenney, February 17, 2011)

Field guides generally show if a species is monomorphic or dimorphic. This is done with little male & female signs (♂♀), for dimorphism. If it says either “adult” or nothing, it’s monomorphic.

The reasons why a species is one or the other are complex. Dissertations and books have been- and still are being – written on the subject.

Four important factors to help understand sexual morphism:
Operational Sex Ratio (OSR): The ratio of the numbers of sexually receptive males to females.
Potential Reproductive Rate (PRR): The offspring production per unit of time each sex would achieve if unlimited mates were available.
Minimum Time Investment (MTI): The shortest amount of time either sex must contribute to produce a fertilized egg.
Parental Time Investment (PTI): The amount of time either sex contributes to parental duties.

For the purposes of this discussion, we’ll use the following definitions:
Breeding duties: All reproduction-related behaviors from courting, mating and egg-laying, through nest-building, incubation of eggs and feeding, protection and education of the young.
Nesting duties: Those activities listed above except mating and egg-laying.

Theoretically, the male and female can have the same MTI, and for many animals that is true. But in birds, because females must produce the large fertilized egg with a protective shell, white and yolk, while the males contributes only tiny sperm, the female’s MTI is always larger. Therefore, if either sex begins to make a larger parental time investment (PTI) than the other, as often happens in evolution, it is almost always the female. This greater (MTI+PTI) investment drives the evolution of the female’s greater choosiness in mate selection, picking – as best she can – the most fit male available. Female choosiness in turn causes males to compete in numerous ways for breeding opportunities. Such competition between males sets up the evolution of variations in ability to sing, gather food, build a nest, fight battles, and especially to display plumage.
[Article: Operational sex ratios & roles, dimorphism, monomorphism]

Allen’s Hummingbird shows extreme sexual dimorphism; female left, male right (Jim Kenney: female September 4, 2009, male January 9, 2007)

Thus it is the female selection of attractive, healthy, large or strong characteristics that shapes the male’s appearance and so produces sexual dimorphism. Because males cannot take on the female’s large initial time investment in egg-laying, males evolve in other ways: nest building, bringing food to the female, territory protection, and predator distraction. His overall parental time and energy investment in these activities may exceed the female’s combined egg production and parental time investment. Further, by performing such necessary duties, the female can concentrate on incubation, etc. Thus a sexual division of labor appears – different roles for females and males.

What does this division of labor have to do with monomorphism and dimorphism? In summary the “rule” which I alluded to at the beginning is:

Greater equality in breeding duties means greater similarity in appearance.

Eggs are always produced by females. In monomorphic species everything else is shared as equally as possible: site-selection, nest-building, incubation of eggs, territory defense, protection, feeding and education of the young. In sexually dimorphic species, duties of nesting and parenting diverge between sexes, with the manner and amount varying widely between species.

This brings up the second part of the “rule”:

The lesser the similarity in appearance, the lesser the involvement in breeding duties by the male, who is the more colorful bird.

When sexual dimorphism is extreme, the male may do nothing beyond inseminating the female. She then performs all the duties of nest-building, incubation of eggs and protection, feeding and education of the young, etc., with no assistance from the male. The avian families of Pheasants, Hummingbirds, Cotingas, Manakins, Birds-of-Paradise and Bowerbirds are filled with such species.

In Part II, we discuss the topics of double-clutching, sexual dimorphism reversal, sexual size dimorphism reversal and polyandry, with illustrative cases.
In Part III, we introduce those polyandrous species that display sexual dimorphism reversal.
[Chuck Almdale]