Birds display an astounding variety of plumage colors and patterns. Some species, like peacocks and birds-of-paradise, have elaborate and brightly colored plumage. Others, like penguins and vultures, have more subdued blacks, whites, and grays. But in some species, males and females look exactly the same – a phenomenon known as monomorphism.
What is monomorphism?
Monomorphism refers to species in which males and females have identical external appearance and plumage coloration. This is in contrast to sexual dichromatism, where males and females differ in coloration. Dichromatism is common in birds – think of the showy peacock and drab peahen. But in monomorphic species like crows, males and females look exactly alike.
Monomorphism may apply to either juveniles and adults having the same plumage (age monomorphism) or to males and females having the same plumage (sexual monomorphism). Sexual monomorphism, where the sexes are indistinguishable by plumage, is estimated to occur in around 10% of all bird species.
Why did monomorphism evolve in some birds?
There are several leading hypotheses for why monomorphic plumage has evolved in certain groups of birds:
Natural selection
In species that need cryptic plumage for camouflage, such as nightjars that roost on the ground, natural selection has favored identical dull plumage in males and females. Bright colors or elaborate plumage would make these birds more obvious to predators.
Low sexual selection
In many monomorphic species, males do not need elaborate plumage to attract mates or defend territories. With little sexual selection pressure driving differentiation, identical plumage persists. For example, many seabirds like gulls and petrels breed at isolated colonies with abundant resources and few territories to defend. Both sexes engage in parental care, further reducing selection pressures.
Need for crypsis during incubation
In species where only one sex incubates eggs, such as phalaropes where males incubate eggs, natural selection has favored cryptic plumage in the incubating sex so eggs and chicks are camouflaged. This leads to monomorphism if the other sex also comes under selective pressure for crypsis.
What bird groups tend to be monomorphic?
Some examples of common bird groups that tend to be monomorphic include:
- Seabirds – Examples: gulls, terns, petrels, albatrosses
- Birds of prey – Examples: hawks, eagles, falcons
- Gamebirds – Examples: grouse, ptarmigan, turkeys, pheasants
- Tropical forest birds – Examples: toucans, parrots, antbirds
- Nocturnal birds – Examples: owls, nightjars
In these groups, natural selection for cryptic plumage and/or reduced sexual selection pressures seem to favor monomorphism. However, there are exceptions in most groups, as some species evolved dichromatism.
Are there benefits to monomorphism?
Monomorphism may confer some advantages to birds beyond camouflage:
- Reduced agonistic behavior – With less difference between males and females, aggressive interactions may be reduced
- Increased parental care from males – Males invest more in care when parenthood is uncertain due to monomorphism
- Lower energy expenditure – Producing elaborate plumage can be energetically costly
However, monomorphism also has some potential disadvantages relative to dichromatism:
- Reduced ability for sex recognition – Can lead to same-sex pairing attempts
- Less effective species recognition – Plumage differences can reinforce species boundaries
- Reduced opportunities for sexual selection – Can limit potential for evolutionary elaboration and adaptation
Examples of monomorphic bird species
Here are some specific examples of widespread monomorphic bird species:
Species | Description |
---|---|
Mallard | Common dabbling duck across North America, Europe, and Asia. Male and female are mottled brown with green iridescent head. |
Canada Goose | Abundant goose with brownish-gray plumage. Males and females honk loudly and are aggressive in defending nest sites. |
American Crow | All-black passerine found throughout North America. Both sexes have black feathers covering the body. |
Black Skimmer | Coastal seabird with black upperparts and white undersides. Males and females have identical plumage coloring. |
Turkey Vulture | Widespread scavenging bird of prey. Females and males have similar black plumage with red heads. |
Are any monomorphic bird species sexually dimorphic in size or display?
While monomorphic birds have identical plumage, some species show subtle sexual dimorphism in size, proportions, displays, or bare parts:
- Many raptors like hawks and eagles have females larger than males, an adaptation allowing females to produce larger eggs.
- Male American goldfinches have brighter bare parts and more vigorous displays than females.
- Male and female plumage is identical in phalaropes, but females are larger and more brightly colored in their bare parts.
- Male blue jays are slightly larger than females on average, with larger crests.
So while monomorphic in plumage, birds may still exhibit sexual differences tuning to their life histories and sexual selection pressures.
How do monomorphic birds recognize each other’s sex?
Since visual signals are unreliable for monomorphic birds, they use non-visual cues for sex recognition and mate choice:
- Behavioral displays – Complex courtship dances or aerial maneuvers
- Vocalizations – Sex-specific calls, like the female blackbird’s chuckle
- Bare parts – Colored skin, wattles, eyes, and beaks
- Size differences – Subtle variation in bulk or proportions
- Genital observation – Direct cloacal inspection during copulation
Research shows monomorphic birds rely more on auditory, olfactory, and tactile signals compared to dimorphic species. But interestingly, some monomorphic birds still retain behavioral displays, suggesting these help reinforce bonds rather than identify sex per se.
Anhinga
A good example is the anhinga. Males and females of this waterbird look identical, both having silver-gray plumage and black wings. Yet males still perform elaborate courtship displays, spreading their wings and snaking their necks while swimming around females.
Conclusion
In summary, monomorphism is thought to arise in birds through natural selection for crypsis and reduced sexual selection pressures for showy plumage. Seabirds, birds of prey, and other groups subject to these pressures often exhibit monomorphism. While monomorphic birds lack sex-specific plumage cues, they have evolved other signals like vocalizations and displays to facilitate mate recognition. Subtle size differences and bare part colors can also play a role. Overall, monomorphism exemplifies the power of natural selection and how adaptation relates to different lifestyles and reproductive strategies in birds.