Sexual selection
Charles Darwin, in The Descent of Man (1871), proposed that some traits spread not because they help an animal survive but because they help it win mates.
The Zoo · New exhibits · Sexual dimorphism in birds
Why the boy duck is colorful
A drake mallard has a glossy green head. The hen beside him is brown and speckled. Same species, very different looks: biologists call it sexual dimorphism.
Displays
Three ways to court
The birds move; they stand still under reduced motion.
Female choice
In many birds the female picks the male, so males with brighter colors, longer plumes or better songs leave more young. Over many generations the trait becomes extreme.
Male competition
Males also fight or bluff each other for territories and mates; size, spurs and bold patches help.
Hidden mothers
A bright bird on the nest is easy for a fox or a raccoon to find. Where the female sits on the eggs alone, natural selection favors camouflage for her.
Honest signals
Bright red and yellow feathers come from carotenoid pigments in food. A cardinal that is very red may be showing that he eats well and is healthy.
Displays and dances
Plumage, dances, songs and gifts: birds-of-paradise and manakins perform on display grounds called leks; bowerbirds build and decorate stages.
Show-offs
Indian peafowl
Pavo cristatus
LCLeast concern
Mallard
Anas platyrhynchos
LCLeast concern
Wood duck
Aix sponsa
LCLeast concern
Mandarin duck
Aix galericulata
LCLeast concern
Northern cardinal
Cardinalis cardinalis
LCLeast concern
Magnificent frigatebird
Fregata magnificens
LCLeast concern
Greater bird-of-paradise
Paradisaea apoda
LCLeast concern
American kestrel
Falco sparverius
LCLeast concern
The exceptions
Role reversal. In phalaropes the female is the brighter bird. She courts the male, may lay eggs for more than one mate, and leaves him to incubate and raise the chicks. The same pattern shows up in jacanas and some shorebirds.
Bigger females. In hawks, falcons and owls the female is usually the larger bird ("reverse size dimorphism"); scientists still debate why.
Ducks, geese and swans
Waterfowl show the whole range. In ducks, pairs often last one season and the drake leaves before the eggs hatch: strong dimorphism, and drakes molt into a dull eclipse plumage after breeding. In geese and swans, pairs usually stay together for years and both parents guard the young: males and females look nearly alike, the male a little larger.
Canada goose
Branta canadensis
LCLeast concern
Mute swan
Cygnus olor
LCLeast concern
Trumpeter swan
Cygnus buccinator
LCLeast concern
Tundra swan
Cygnus columbianus
LCLeast concern
Domestic duck
Anas platyrhynchos domesticus
DOMDomestic
| Swan | Bill | Native to North America | Note |
|---|---|---|---|
| Mute swan | Orange with a black knob | No (from Europe) | Restricted in Florida; crowds out native birds |
| Trumpeter swan | All black | Yes | Heaviest native waterbird; nearly lost by the 1930s |
| Tundra swan | Black, often a yellow spot | Yes | Nests on Arctic tundra |
Grades K–5
Why is the boy duck so colorful?
The mother duck sits on the eggs, so she is brown like the grass to stay hidden from foxes. The father duck is bright so the mother duck notices him. Geese and swans share the work, so moms and dads look alike!
For classes
Grades K-2
The Nest: Watch one birdPreview Grades 3-5
The Bird Aviary: Why are they different colours?Preview Grades 6-8
Heredity and Evolution: Darwin and WallacePreview Drawer: Zoo: Why the boy duck is colorful QR YBQJJ6
Quiz: Why the boy duck is colorful quiz · 7 questions
Next door: Aviary zone · Children's zoo (ducks) · Lamby: Feathers and eggs
Sources
- Darwin, C. (1871) The Descent of Man, and Selection in Relation to Sex (Darwin Online)
- Cornell Lab of Ornithology: Mallard
- Cornell Lab: Red-necked Phalarope
- Cornell Lab: Birds-of-Paradise Project
- Hill, G.E. (1991) Plumage coloration is a sexually selected indicator of male quality. Nature
- FWC: Mute swans in Florida
Checked November 2026. Figures marked (est.) are estimates; statuses are flagged in the catalog for re-checking against the live IUCN Red List before acceptance.