Hummingbirds are known for their iridescent plumage and ability to hover and fly backwards. They have long slender bills that are perfect for sipping nectar from flowers. One distinguishing feature of many hummingbird species is a small white spot behind each eye. This spot is known as a postocular spot or “eye ring.” In this article, we will explore what causes this white spot on hummingbirds and why it exists.
The postocular spot is created by specialized feathers called plumules that lack pigment, revealing the white downy base. It is present in over half of all hummingbird species, but is most prominent in North American varieties like the Ruby-throated Hummingbird and Rufous Hummingbird. Researchers believe the main purpose of the eye ring is communication. The high contrast of the white spot against the otherwise colorful plumage helps hummingbirds identify and interact with each other. It may also serve as a target during aggressive interactions or courtship displays. Understanding the function and purpose of this marking can provide insight into hummingbird behavior and ecology.
What Causes the White Spot?
The white spot behind a hummingbird’s eye is created by a lack of pigment in the specialized feathers called plumules that grow in that location. Most of a hummingbird’s colorful plumage is caused by pigments like melanins and carotenoids within the feather structure. However, the plumules that emerge behind the eyes lack these pigments, revealing the white downy base of the feather. This results in the conspicuous postocular spot.
Within the avian family, pigment patterns are precisely controlled by gene expression. The development of pigmentation in feathers is complex and involves many different genes. In the case of the hummingbird eye ring, genes that control pigment deposition are selectively turned off only in the specialized plumules behind the eyes. The genetic mechanisms regulating this precise pattern are not fully understood but likely involve signaling molecules that inhibit pigment production in specific feather follicles. More research is needed to unravel the genetic basis of this marking.
Prevalence Across Hummingbird Species
The white postocular spot is very common among hummingbird species, though not universal. One analysis found the spot present in over 65% of extant hummingbird species. However, there is variation in the size, shape, and symmetry of the spot across different types of hummingbirds.
Some species like the Ruby-throated Hummingbird and Allen’s Hummingbird display very pronounced and symmetrical eye rings. Others like the Berylline Hummingbird have smaller less defined spots. A few unusual species, like the Violet-tailed Sylph, lack an eye ring altogether. There can also be significant individual variation within a species. The reasons behind this diversity in eye ring morphology are not fully clear but may relate to different selective factors across habitats and evolutionary history.
Broadly speaking, hummingbirds in the subfamily Trochilinae of North and South America tend to have more well-defined eye rings than their Old World relatives in the Phaethornithinae subfamily of Africa, Asia and Europe. However, even within the new world Trochilinae, there is considerable variation. Examples of Trochilinae species without postocular spots include the Violet-bellied Hummingbird and White-vented Plumeleteer. More phylogenetic analysis and comparative study across hummingbird lineages may reveal evolutionary patterns behind the distributions of this trait.
Purpose and Function
Researchers propose several hypotheses for why the white postocular spot evolved and how it benefits hummingbirds:
Facial Recognition
The high contrast spot aids in facial recognition and individual identification. Hummingbirds have relatively small brain volumes compared to their body size, and studies show they have more difficulty with cognitive tasks like memory and learning compared to songbirds. The eye ring provides a simple visual cue to help hummingbirds distinguish and remember individuals, which is advantageous for maintaining territories and complex social relationships.
Target for Aggression
When defending territory or competing for mates, male hummingbirds engage in aerial battles and aggressive displays. The eye ring serves as a focal point that opponents can attack during these contests. This may limit damage to crucial areas like the head, bill, and body. Targeting the expendable feathers around the eyes provides a way to signal aggression and establish dominance without causing serious harm.
Courtship Display
During courtship, the high contrast spot draws visual attention and accentuates head movements and orientation. By emphasizing this area, the eye ring allows hummingbirds to better communicate subtle visual cues associated with mating behaviors. This allows the birds to effectively signal their reproductive status and fitness to potential mates.
Camouflage Disruption
Some scientists argue the eye spot works to disrupt the bird’s outline and interfere with predators’ ability to recognize a hummingbird as potential prey. This is a form of disruptive coloration that breaks up the body’s telltale shape to make it harder to parse against the background environment. However, others contest this idea since the white spot is very small relative to the hummingbird’s overall size.
More experimental research manipulating the variable presence and absence of eye rings under controlled conditions can help clarify their adaptive significance. Analyzing genes associated with eye ring development across species may also provide evolutionary insights into how and why this trait arose. The trait remains incompletely understood but potentially serves multiple purposes related to communication, signaling, and perception.
Differences Between Males and Females
There are some subtle but notable differences between male and female hummingbirds when it comes to the white postocular spot:
Size
In species where males and females both exhibit eye rings, the spots tend to be larger in males on average. This trend aligns with the hypothesis that the spot plays a role in aggressive displays and mate attraction. The larger spot in males may amplify these signaling functions.
Color
Some species show sexual dichromatism where males have iridescent gorgets (color patches on the throat) while the female gorgets are drab white. In these species, the male eye ring feathers may contain more iridescent sheen and colorful iridescence than in females. This further enhances the visual prominence of the spot in courting males.
Symmetry
Studies of traits like the Swordbill Hummingbird show the white spot behind the eyes tends to be more symmetrical in males compared to females. Greater symmetry may correlate with male fitness and be used as a metric for selecting vigorous mates.
Presence/Absence
A few species like the Booted Racket-tail lack eye rings in females but contain them in mature males. Development of the spot may correlate with sexual maturity in these cases and help indicate reproductive status.
Overall, the subtle differences in size, color, symmetry, and presence between sexes supports the sexual selection and communication hypotheses regarding the functional role of the eye ring. More comparative studies quantifying sexual dimorphism across species can provide greater insights into the evolutionary origins and advantages conferred by this trait.
Changes Between Juveniles and Adults
The white spot behind a hummingbird’s eyes can change significantly as the birds mature from juveniles into adults:
Size Increase
In many species, the postocular spot starts out relatively small in young hatchlings and increases in size as the birds mature into adults. For example, juvenile Allen’s Hummingbirds initially have minute spots that may be barely perceptible. By adulthood, the spot expands to a well-defined and conspicuous ring.
Color Change
Some juvenile hummingbirds have buff or light gray postocular spots that darken with age. In species like the Rufous Hummingbird, the light gray spot eventually transitions to clean white in mature adults. Such color changes may correlate with nutritional status, feather quality, and overall fitness.
Symmetry Improvement
Like other sexual ornaments, the eye rings tend to become more symmetrical as birds reach adulthood. There is often considerable asymmetry in the spots of young fledglings that even out and improve by the first year of life. This may reflect motor development and coordination in controlling feather growth.
Later Onset
In a few species, juveniles entirely lack eye rings which only emerge later in ontogeny around the first molt into adult plumage. For example, young male Booted Racket-tails do not have spots but quickly develop the trait when reaching sexual maturity. The timing of spot onset seems strategically linked to reproductive signaling.
Tracking developmental changes in the postocular spots can provide insight into the ontogeny of feather follicles. Changes likely represent a maturation of the genetic mechanisms controlling regional pigmentation. This highlights the complex intersection between genetics, development, behavior and evolution reflected in this one small trait.
Geographic Variation
Like many avian plumage traits, the hummingbird eye ring shows geographic variation between different subspecies and populations:
| Species | Population | Eye Ring Variation |
|---|---|---|
| Allen’s Hummingbird | Southern California | Thin white ring |
| Pacific Northwest | Thicker ring | |
| Rufous Hummingbird | Alaska to Northwest | Reddish tone |
| California | Paler white | |
| Calliope Hummingbird | Far northern range | Indistinct |
| Southern range | Brighter, bolder |
This regional variation suggests local adaptations based on selective factors like habitat and pressures from competitors or predators. For example, redder hued spots in northern Rufous Hummingbirds may provide better contrast in evergreen forests. Hotter southern climates may favor reduced spot dimensions to mitigate feather abrasion or soiling. Environmental factors likely shape the variation, but sexual selection and drift could also play a role. More sampling across contact zones between phenotypes may shed light on the potential drivers and evolutionary mechanisms at play. Tracking changes over time could also reveal if shifts correlate to climate change.
Condition Dependence
The expression of the postocular spot depends on the health and condition of individual hummingbirds:
Nutrition
Adequate nutrition is required to generate healthy, bright white feathers for the spot. Impoverished or deficient diets can result in thinner, frayed, or off-colored spots. Abundant nectar availability allows hummingbirds to achieve optimal plumage.
Parasites
Parasitic infections that cause inflammation around growing feathers or deplete resources can disrupt spot development. For example, the parasitic botfly Philornis can infest nestlings and create misshapen or asymmetrical spots in adults. Good early life health is vital.
Injuries
Physical traumas like collisions may damage the specialized feathers behind the eyes. This can result in broken, bent, or lost feathers altering the eye ring’s appearance. The spot is particularly prone to wear and tear during territorial disputes when rivals directly peck and claw at that area.
Molt
Hummingbirds undergo an annual molt when new feathers replace old ones. During this process, the postocular spot may appear flecked, variable, or incomplete until the new plumules finish growing in. The pace and progression of molting impacts temporary spot condition.
Age
As hummingbirds reach the limits of old age and senescence, their eye rings deteriorate along with the rest of their plumage. The spots may fade, thin, or become irregular in elderly birds.
By controlling for nutrition, health, and environmental factors, researchers can disentangle developmental and genetic influences on eye ring expression. This can shed light on what aspects of the trait are fixed versus plastic and environmentally modifiable.
Role in Taxonomy and Classification
The white postocular spot has served as a helpful taxonomic feature used to classify different hummingbird groups, especially among the diverse South American species:
Trochilus group
In 1957, ornithologist Robert Ridgeway designated the “Trochilus” group containing over 100 New World hummingbird species all distinguished by their conspicuous white postocular spots. This served as a useful anchoring trait for grouping diverse taxa.
Oreotrochilus genus
In 1973, Dr. Stiles and colleagues proposed separating out 23 Andean hummingbird species into the new genus “Oreotrochilus.” One of the key traits used to distinguish and validate this new lineage was the shared white postocular spot among its member species.
Topaza group
Within the multi-barbets, traditional classification included the Topazes within the larger Jacobin group. In 1999, Dr. Schuchmann proposed dividing them into a separate Topaza lineage defined by traits including the lack of a marked postocular spot.
Discovering new species
Comparing head plumage and postocular patterns has helped researchers identify new hummingbird taxa as distinct from close relatives. Examples include the recent splitting of the Blue-vented Hummingbird from the Magnificent Hummingbird in 2017.
Careful examination of subtle differences in this eye feather marking continues to aid biologists in unraveling the evolutionary trees and taxonomic relationships among hummingbird species. It remains an invaluable visual clue for classifying groups and discriminating closely allied taxa across this highly diverse avian radiation.
Role in Hummingbird Identification
For avid birdwatchers, the presence, shape, and color of the postocular spot aids in identifying different hummingbird species in the field:
Distinguishing look-alikes
The spot helps differentiate between similar sister species like Calypte anna’s and Calypte costae where males have identical gorgets. C. anna’s has an eyering connecting to the gorget, while C. costae’s is isolated.
Diagnosing age
Subtle spot size and color changes pinpoint juvenile versus adult birds. For example, fledgling Allen’s Hummingbirds have tiny dark spots compared to the bold white ring in adults.
Confirming sex
Males and females are monomorphic (appear identical) in some species. However, sex can be inferred from slight postocular spot variations.
Eliminating confusion
The obvious spot distinguishes hummingbirds from similarly sized but unrelated birds like bushtits or vireos. This prevents misidentifications.
Estimating range
Knowing which species have spots in a given region narrows ID options. For example, a spotted hummer on the East Coast must be a Ruby-throated.
With hundreds of kinds clustered into small geographic areas, anything that simplifies identification helps birders distinguish the array of buzzing colors. Focusing on subtle eye ring differences allows experts and enthusiasts to readily put a name to these feisty, fast-flying species. The usefulness of the spot for recognition speaks to its evolutionary significance.
Conclusion
The tiny white spot behind a hummingbird’s eye may seem like an inconsequential fleck of plumage. However, this postocular marking represents a remarkable example of selective pressures and genetic mechanisms conspiring to create a beneficial trait. Sexual selection, territorial defense, mate signaling, species identification, and other factors all likely contributed to the evolution and persistence of the eye ring across diverse hummingbird taxa. Variation in size, shape, color, and symmetry of this spot reveals insights into environmental adaptation, developmental pathways, and evolutionary history. Though small in size, the functions and implications of this specialized plumage patch loom large. Careful study of such a refined and well-defined trait continues to inform ornithology and enrich our understanding of natural selection. The hummingbird eye ring illustrates how even a diminutive patch of feathers can speak volumes about the forces that shape biodiversity.
