Sexual dimorphism refers to physical differences between males and females of the same species. In birds, this often manifests in differences in size, plumage coloration, ornamentation, and courtship displays between the sexes. There are many avian species that exhibit sexual dimorphism to varying degrees.
Common Examples of Sexually Dimorphic Birds
Here are some common examples of sexually dimorphic bird species:
- Peafowl – Male peafowl (peacocks) have brightly colored train feathers used for courtship displays, while females (peahens) have drab brown plumage.
- Birds of paradise – Males have elaborate, brightly colored plumage used in mating displays, while females are plain and nondescript.
- Cardinals – Male cardinals are vivid red, while females are tan/olive colored.
- Mallards – Male mallards have a bright iridescent green head, while females are mottled brown.
- Eclectus parrots – Males are bright emerald green while females are vibrant red and purple.
- Phalaropes – Females are larger and more colorful than males.
- Lories and lorikeets – Males and females can look strikingly different in color and markings.
Why does sexual dimorphism occur in birds?
There are several theories as to why sexual dimorphism is so prevalent in birds:
- Female choice – In many species, females select males based on flashy plumage and courtship displays, driving the evolution of elaborate male traits.
- Sexual selection – Competition between males to attract female mates leads to the evolution of exaggerated male characteristics.
- Different reproductive roles – Dimorphism arises from differences in reproductive investment between males and females. Females devote energy to producing eggs, while males compete to fertilize.
- Niche partitioning – Physical differences reduce competition between the sexes for food and other resources.
- Camouflage – Drab female plumage provides camouflage from predators while incubating eggs and caring for young.
Examples of Sexually Dimorphic Bird Groups
Here are some bird groups that demonstrate strong sexual dimorphism:
Birds of Paradise
There are over 40 species of birds of paradise, famous for the elaborate plumage of males. Females are plain brown in color. Well-known examples include:
- King bird-of-paradise – Males have a bright red and white plumage and long wire-like flank feathers.
- Wilson’s bird-of-paradise – Males have a striking yellow crown and green breast shields.
- Twelve-wired bird-of-paradise – Males have many wire-like flank plumes that can be moved independently.
Hummingbirds
Male hummingbirds are often brightly colored with iridescent plumage, used to attract females. Females are much plainer in appearance. Some examples include:
- Anna’s hummingbird – Males have rose-pink throats and crowns, females are gray-green.
- Costa’s hummingbird – Males have vivid purple crowns, females are dull gray.
- Rufous hummingbird – Males are brightly orange, females are green.
Pheasants and Peafowl
Male pheasants and peafowl grow long, ornate tail feathers for courtship displays. Females lack such adornments. Well-known species include:
- Peacocks – Males have a long train of iridescent blue and green tail feathers.
- Indian peafowl – Males have an elaborate train covered in eye-like spots.
- Golden pheasant – Males have vivid red plumage with blue facial skin.
- Reeve’s pheasant – Males have long, fluffy tail feathers striped black and white.
Ducks and Geese
Male ducks and geese tend to have brightly colored plumage patterns compared to their female counterparts. Examples include:
- Mallard – Males have an iridescent green head, females mottled brown.
- Mandarin duck – Males have bright orange “sails” on the side of their face.
- Wood duck – Males have colorful crests and plumage, females are brown-speckled.
- Canada goose – Males have a black head and neck, females have a brown neck and head.
Birds of Prey
In birds of prey, females are typically larger than males. This is likely because their size gives them an advantage when incubating eggs and catching prey. Examples include:
- Bald eagle – females are around 25% larger than males.
- Northern goshawk – females are up to 60% heavier than males.
- Common buzzard – females are noticeably larger and broader winged.
Measuring Sexual Dimorphism
Ornithologists use different measures to quantify the degree of sexual dimorphism in bird species:
- Size dimorphism: Comparing male and female body mass or length measurements.
- Plumage dichromatism: Scoring differences in male and female plumage coloration and patterning.
- Ornamentation: Comparing the size of male ornamental traits like wattles, combs, and feather trains relative to females.
- Display rates: Measuring differences in male courtship display behaviors compared to females.
By using statistical methods to analyze these dimorphisms, researchers can better understand the evolutionary forces driving them.
Conclusion
Sexual selection has led to amazing diversity in avian sexual dimorphisms. From the elaborate trains of peacocks, to the minuscule hummingbirds, to large powerful birds of prey, males and females have evolved distinct specializations. These differences likely arose due to reproductive pressures – males competing for female attention, and females dedicating energy to egg-laying and rearing young. Overall, sexual dimorphism allows males and females to effectively play their separate roles in reproduction.