Ruby-throated Hummingbirds (Archilochus colubris) are the most common and widely distributed hummingbird species in eastern North America. They are sexually dimorphic, meaning males and females have distinct plumage differences. This allows for easy differentiation between the sexes.
Male Ruby-throated Hummingbird
Adult male Ruby-throated Hummingbirds have vibrant iridescent ruby-red throats, from which their name is derived. The ruby-red throat is a highly reflective patch that appears black at some angles but shimmers brightly red at others. It is used for courtship displays to attract females.
Apart from the ruby throat, the rest of the male’s plumage is predominately green. The crown and back are metallic green, while the underside is dull grayish-green. The tail is black with an iridescent purplish-red spot at the base of each outer tail feather. The wingtips are also black.
Males measure 7–9 cm in length and 2–3 grams in weight. Their wingspan is 3.5–4.3 inches. Their slender bill is straight and very long compared to body size, enabling them to reach nectar at the base of long tubular flowers.
Female Ruby-throated Hummingbird
Adult female Ruby-throated Hummingbirds lack the distinctive ruby red throat patch of the males. Instead, they have white throats and chins. In poor lighting conditions, the white throat may appear dull greyish.
Apart from the throat, the female’s plumage is very similar to the male’s in coloration. Their crown and back are metallic green, while the underside is dull grayish green. The black tail with iridescent reddish-purple spot on the outer tail feathers is also the same.
Females are slightly smaller than males, measuring 7–9 cm in length with a 2.5–3.5 inch wingspan. Their weight ranges from 2–3 grams. As in males, the bill is long, straight and slender.
Juvenile Ruby-throated Hummingbirds
Hatchlings of both sexes resemble adult females in having white throats. Their plumage is overall much duller and lacks the vibrant iridescent colors of adults. The back appears dull grey-green rather than metallic green. The reddish-purple spots on the tail are also less distinct.
Juveniles molt into adult plumage at around 3 months of age. Young males start to acquire the red throat feathers and more colorful iridescent plumage leading up to their first winter.
Differences in Behavior
In addition to physical appearance, males and female Ruby-throated Hummingbirds exhibit some different behaviors especially related to breeding.
Males are highly territorial. They defend feeding territories from other males and establish courtship territories to attract females. Their courtship displays include flying in U-shaped or J-shaped dives across the territory.
Females alone build the nest, incubate the eggs and care for the chicks. They are very cryptic while nesting and tend to visit flowers along woodland edges rather than out in the open.
Males do not participate at all in nesting duties. During courtship, the male will attempt to mate with the female while she is gathering nesting material. He does not associate with the female after mating.
Appearance Differences Summary
Here is a quick overview of the key physical differences between male and female Ruby-throated Hummingbirds:
| Feature | Male | Female |
|---|---|---|
| Throat | Ruby red iridescent | White |
| Crown | Metallic green | Metallic green |
| Back | Metallic green | Metallic green |
| Underside | Grayish green | Grayish green |
| Tail spots | Iridescent reddish-purple | Iridescent reddish-purple |
| Size | 7-9 cm long | Slightly smaller at 7-9 cm |
Identification Tips
Here are some key points to help identify whether a Ruby-throated Hummingbird is male or female:
- Look at the throat – only adult males have a distinctive ruby red throat
- Females and juveniles of both sexes have white throats
- Body plumage color is otherwise very similar between the sexes – metallic green crown and back, grayish green underside
- Females are slightly smaller than males on average
- Juveniles have duller overall plumage than adult birds of both sexes
The ruby red throat is by far the most reliable feature for identifying adult male Ruby-throated Hummingbirds. Females and young birds can be a challenge to differentiate, but with practice the subtle size and color differences can be discerned.
Geographic Variation
The plumage of Ruby-throated Hummingbirds is very consistent across their wide geographic breeding range in eastern and central North America. No distinct subspecies are recognized currently.
However, there is a small degree of individual variation, primarily among females, in the darkness or lightness of the white throat patch. Females breeding in the northern parts of the range tend to have darker grey-tinged throats compared to females in more southern locales.
Males also show slight variation in the extent of ruby red coloring on the throat between individuals. But this minor variation does not delineate distinct subspecies or geographic forms.
Western Ruby-throats
In recent decades, Ruby-throated Hummingbirds have expanded their breeding range westward into areas along the Pacific coast such as British Columbia and Washington where they were previously just migrants. These west coast breeders look indistinguishable from eastern Ruby-throats.
There is no evidence that western fringe populations differ in appearance from birds in the ancestral eastern breeding grounds. The species remains monotypic across its increasingly transcontinental breeding distribution.
Evolution of Sexual Dimorphism
The evolutionary cause of differing male and female plumage within bird species is called sexual selection. In sexually dimorphic species like hummingbirds, males have evolved elaborate decorative plumage to attract females.
In Ruby-throated Hummingbirds, the males likely evolved metallic plumage colors to enhance visibility. The ruby-red throat plumage probably functions specifically as a courtship signal to females, given that it is only briefly displayed during courtship displays.
Research indicates the throats of male Ruby-throats are highly reflective of light, causing them to flash brightly red during dive displays towards females. This stimulates females and indicates male fitness.
Females developed more cryptic plumage without bright ornamentation since they alone incubate eggs and brood young. Camouflaged plumage provides selective advantage by reducing visibility to predators at the nest.
Role in Identification
The pronounced sexual dimorphism in Ruby-throated Hummingbirds aids greatly in field identification and bird banding studies. The bright red throat patch allows researchers to easily determine the sex of birds captured in mist nets for banding.
During systematic bird surveys such as breeding bird atlases, observers record the sex of each Ruby-throated Hummingbird observed based on presence or absence of the red throat. This provides valuable data on sex ratios and population demographics.
For birders, knowing how to distinguish males and females is an essential skill for properly identifying Ruby-throated Hummingbirds in the field. Only the males display the brilliant red throat, so a view of the throat is needed for certain identification of males.
Evolutionary Advantages
Sexual dimorphism in Ruby-throated Hummingbirds has likely evolved through sexual selection for the following reasons:
- Brightly colored plumage helps male hummingbirds visibility to females during courtship displays
- The male’s red throat signals health and fitness as a mate choice cue for females
- Camouflaged female plumage provides adaptive advantage at the nest
- Distinct plumage differentiates mating roles and behaviors between the sexes
The differing reproductive duties of males and females seems to be the main evolutionary driver of divergent plumage characteristics. Males are selected for flashy displays, while female plumage adapts for nesting success.
Comparison to Other Hummingbird Species
Pronounced sexual dimorphism as seen in Ruby-throated Hummingbirds is common among hummingbird species. Here is how Ruby-throated Hummingbird plumage compares to some other sexually dimorphic hummingbirds:
Anna’s Hummingbird
– Males have iridescent rose-pink throats and crowns, females have grey-green plumage
Costa’s Hummingbird
– Males have vivid purple crown and throat, females are mostly grey-green
Broad-tailed Hummingbird
– Males have iridescent fuchsia throat, females have pale grey-white throats
The consistent theme across species is elaborate male ornamentation versus camouflaged female plumage. This pattern of divergence suggests sexual selection drives common plumage differences in hummingbirds.
Male Ornamentation in Other Bird Species
Pronounced plumage differences between the sexes are not unique to hummingbirds. Dimorphism driven by sexual selection is also found in:
- Peacocks – males have extravagant tail plumes, females are plain brown
- Birds of paradise – dramatically ornamented males, camouflaged females
- Cardinals – bright red males, tan females
- Mallards – brightly colored males, mottled brown females
Across diverse bird families, males frequently evolve showy traits for courtship, while natural selection acts on females to blend in while nesting.
Conclusion
In conclusion, male and female Ruby-throated Hummingbirds can be readily distinguished by pronounced differences in plumage resulting from sexual dimorphism. The vibrant red throats and colorful iridescence of males plays a role in courtship. The more cryptic plumage of females provides camouflage while nesting. These differences suggest evolutionary forces of sexual selection acting divergently on the sexes based on their breeding roles.
