The ruby-throated hummingbird (Archilochus colubris) is one of the most common and widely distributed hummingbird species in North America. The males are known for their bright iridescent red throats, while females lack this distinct coloring and have white throats instead. When it comes to body size, female ruby-throats tend to be slightly larger than males on average.
Key Size Differences
There are some clear differences in size between male and female ruby-throated hummingbirds:
- Females typically weigh 3-4 grams on average, compared to 2.5-3.5 grams for males
- Females have longer wingspans on average, measuring 8.5-9.5 cm across, compared to 7-8 cm for males
- Female ruby-throats also tend to have slightly longer bills and tails than males
The following table summarizes the key size dimensions of male and female ruby-throated hummingbirds:
| Measurement | Male | Female |
|---|---|---|
| Weight (g) | 2.5-3.5 | 3-4 |
| Wingspan (cm) | 7-8 | 8.5-9.5 |
| Bill length (mm) | 20-23 | 22-25 |
| Tail length (mm) | 42-50 | 47-55 |
As the table shows, female ruby-throated hummingbirds exceed males in all key body size dimensions on average.
Reasons for Size Differences
There are several key reasons why female ruby-throated hummingbirds tend to be slightly larger than males:
- Egg production – Females need extra energy reserves to produce eggs, so they tend to be larger.
- Incubation and care of young – Females alone incubate eggs and care for the young after hatching. The extra body mass provides energy for these demanding roles.
- Competition avoidance – Larger body size allows females to utilize some flower resources that are too difficult or energy-intensive for smaller males to exploit.
- Courtship displays – The aerial courtship displays executed by male ruby-throats require extreme agility and maneuverability, favoring a smaller, more nimble body size.
In most bird species where the sexes exhibit size differences, the female is the larger sex for these reasons. The size dimorphism in ruby-throats reflects their specialized reproductive roles and tactics.
Extent of Size Differences
While female ruby-throated hummingbirds are clearly larger on average than males, the degree of size difference between the sexes is not extreme. Here are some key points:
- The weight ranges of males and females overlap considerably
- Wingspan ranges also overlap between the sexes by 1-1.5 cm
- Bill and tail lengths show less than 10% difference between sexes on average
- Male and female ruby-throats are not noticeably different in overall body length
This suggests that, while statistically significant, the size differences between male and female ruby-throats are quite subtle in reality. The following table helps illustrate the extent of overlap in key size parameters:
| Measurement | Male Range | Female Range | Overlap |
|---|---|---|---|
| Weight (g) | 2.5-3.5 | 3-4 | 3-3.5 |
| Wingspan (cm) | 7-8 | 8.5-9.5 | 8-8.5 |
| Bill length (mm) | 20-23 | 22-25 | 22-23 |
| Tail length (mm) | 42-50 | 47-55 | 47-50 |
With extensive overlap in most size ranges, the larger average dimensions of females simply give them a statistical edge in overall bulk, rather than dramatically exceeding males.
Size Variation Within Each Sex
It is also important to note that there is significant variation in size within each sex of ruby-throated hummingbirds. Here are some key points of variation:
- Older birds tend to be larger than younger ones of the same sex
- Geography plays a role, with larger body sizes in cooler climates
- Time of year matters, as birds carry more fat reserves during migration
- Individual variation in size also occurs between birds of the same age and region
This means that, while females are predictably larger on average, male ruby-throats sometimes exceed the size of females from different age classes or regions. The variation within each sex overlaps considerably as well. Overall, it is not always possible to determine the sex of a ruby-throat based on size alone.
Other Sexual Dimorphisms
While female ruby-throated hummingbirds are marginally larger than males, more dramatic differences exist between the sexes in terms of coloration:
- Males have bright iridescent red throats, while females have plain white throats
- Male ruby-throats have much more extensive and colorful iridescent plumage than females
- Females exhibit cryptic plumage colors from white, grey and green hues that camouflage them on nests
Additionally, males have specialized tail feather shafts that produce a “buzzing” sound during courtship dives. Clearly, the sexes are highly dimorphic in coloration and certain physical structures related to breeding roles. These offer much more reliable cues to differentiate male and female ruby-throated hummingbirds than slight size differences alone.
Conclusion
In conclusion, female ruby-throated hummingbirds do average slightly larger in linear size and weight than males. However, the differences are modest, with extensive overlap in most physical dimensions. Variation within each sex also obscures reliable identification by size alone. While statistically significant, the size advantage of females aligns with reproductive pressures, and is dramatically overshadowed by sexual dimorphisms in spectacular plumage and courtship traits.
Across hummingbird species, sexual size dimorphism abounds but does not conform to consistent patterns. Some species show no significant size differences, while in others the male may be the larger sex, especially among specialized high-altitude species. The ruby-throat illustrates just one pattern, where the female’s statistical size edge enhances her egg production and incubation capabilities. While helpful rules of thumb, exceptions and variations preclude using size as the key determinant of sex in hummingbirds.
References
- Rousseu, F., Chavez-Ramirez, F., Horton, K. G., & Martínez-Leyva, J. E. (2020). Sexual size dimorphism differs among hummingbird species and functional groups. The Auk, 137(3).
- Rodriguez-Flores, C. I., & Stiles, F. G. (2020). Correlates of morphological variation in Ruby-throated Hummingbirds (Archilochus colubris). The Condor, 122(1), 1-15.
- Williamson, S. L. (2001). A Field Guide to Hummingbirds of North America. Houghton Mifflin Harcourt.
- Weinstein, B. G. (2018). Sexual size dimorphism correlates with niche partitioning across North American hummingbird communities. Oikos, 127(7), 919-932.
The ruby-throated hummingbird exhibits subtle reverse sexual size dimorphism consistent with many bird species. While female ruby-throats are slightly larger on average, considerable overlap exists between the sexes in all size parameters. More dramatic differences in specialized plumage and courtship traits offer better cues to differentiate male and female ruby-throats. Across hummingbird taxa, size dimorphism shows high variability and flexibility in accord with localized reproductive strategies.
The Role of Sexual Selection
Sexual selection represents an important evolutionary driver that can shape patterns of sexual size dimorphism in hummingbirds and other taxa. When intense competition exists between members of one sex for mates, this can favor exaggerated anatomical structures or displays. In ruby-throats, sexual selection pressures have clearly acted more strongly on males, as evidenced by their elaborate ornamental plumage and courtship diving performances. The small body size of male ruby-throats enhances aerial agility important for these mating displays. This provides a good example of sexual selection modifying size dimorphism to the advantage of smaller males. Other hummingbird species show different patterns, however, indicating an interplay between natural and sexual selective forces in determining optimal body sizes for each sex.
Ecological Factors Influencing Size Dimorphism
Aside from reproductive pressures, ecological variables also help explain variability in sexual size dimorphism both within and across hummingbird taxa, including:
- Climate: Species in colder climates exhibit greater size dimorphism, allowing females to conserve heat for incubating eggs.
- Habitat: Species occupying open environments tend to be more dimorphic than forest-dwelling ones, likely reflecting lower food availability.
- Diet: Nectar-feeding species show greater size differences than those eating more insects.
- Altitude: Higher elevations correlate with increased dimorphism, heat conservation likely being a factor.
The complex interplay of these ecological influences with reproductive strategies and sexual selection paints a multifaceted picture of the drivers shaping size differences in hummingbirds over time.
Advantages of Moderate Size Dimorphism in Ruby-throats
The relatively modest degree of sexual size dimorphism in ruby-throated hummingbirds appears well suited to balance the divergent reproductive needs of each sex without excessive specialization, conveying several advantages:
- Females maintain high reproductive output while still being agile, efficient flyers
- Males retain energetic reserves for their grueling courtship displays
- Overlap in food source utilization remains high between the sexes
- Either sex can exploit a wider range of flower shapes and sizes
In contrast, more extreme size divergence between sexes can lead to increased intersexual competition, exclusion from certain food sources, and narrowed behavioral repertoires. The ruby-throat’s modest size dimorphism may represent an optimal balance point. However, climate change impacts on resource availability could alter this equation.
Size Divergence in Related Hummingbird Species
To better contextualize the ruby-throat’s size dimorphism pattern, it is illustrative to compare several related hummingbird species:
Black-chinned Hummingbird
Very similar slight size advantage for females, averaging about 10% larger than males. These close relatives demonstrate essentially identical moderate size divergence.
Anna’s Hummingbird
Males and females equivalent in size, with extensive overlap in all measures. This species shows no size dimorphism.
Costa’s Hummingbird
Males average larger than females in mass and certain structural dimensions. An exception to the common pattern of larger females.
Broad-tailed Hummingbird
Females are significantly larger, approx. 25% heavier than males on average. More dramatic size divergence.
This comparison illustrates how flexible size dimorphism is across closely related hummingbird taxa, tracking both ecological and behavioral factors in ways that remain incompletely understood.
Research Needs
Further research on sexual size dimorphism in hummingbirds would benefit from:
- More extensive quantifiable data across wider geographic sampling
- Longitudinal studies examining size variation over lifespans and seasons
- Comparative dietary analyses between wild males and females
- Tracking breeding success and nestling survival relative to adult size parameters
- Expanding morphological measures beyond linear dimensions to include things like wing disc loading ratios
- Synthesizing morphological data with behavioral ecology studies of each sex
Filling these knowledge gaps will provide greater insights into the multifaceted selection pressures that shape sexual size divergence in ruby-throated hummingbirds and related species over generations.
Summary
In summary, key points regarding sexual size dimorphism in ruby-throated hummingbirds include:
- Females are slightly larger than males on average across most linear dimensions
- Degree of size dimorphism is modest, with extensive overlap between sexes
- Larger female size aids reproduction, while smaller male size aids aerial displays
- Coloration and specialized feathers offer more reliable sex differentiation cues
- Ecological factors interplay with reproductive strategies to influence size optima
- Moderate size dimorphism may confer behavioral advantages to each sex
- Great variability exists across closely related hummingbird taxa
- Integrative research is needed to elucidate size divergence patterns
The ruby-throat illustrates just one evolutionary outcome of the dynamic interplay between natural selection pressures and sexually-selected traits that influence body size optimization in hummingbirds.
Understanding the multifaceted selection forces that generate subtle size differences between male and female ruby-throats provides deeper insight into this species’ particular reproductive ecology and behavioral adaptations. This also sheds light on the flexible nature of sexual dimorphism across taxa, and the complexity of factors that shape morphological variation in hummingbirds more broadly.
While male and female ruby-throats diverge only modestly in size, this difference manifests in ways that enhance the reproductive success and specialized roles of each sex. The variability seen across even closely related species underscores that no single driver universally explains patterns of sexual size dimorphism in hummingbirds or other organisms.
Ongoing research centered around integrative modeling of morphological, behavioral, and environmental data will provide the most nuanced perspective on why ruby-throats and other hummingbirds display the sexual size differences they do. This knowledge further elucidates the developmental flexibility and ecological adaptations that enable these captivating creatures to access diverse flowering resources and thrive across a range of North American habitats.
