Hummingbirds are known for their ability to hover and fly backwards as well as their beautiful iridescent feathers. But if you look closely at a hummingbird’s face, you may notice a small white spot behind each eye. This spot is an important part of the hummingbird’s anatomy and serves an essential function. In this article, we’ll explore what that white spot is, how it works, and why it’s so important for hummingbirds.
What is the white spot?
The white spot behind a hummingbird’s eye is called the pecten or pecten oculi. The pecten is a crescent-shaped structure made up of folded tissue that contains blood vessels. It is located within the vitreous humor, the gel-like substance that fills the inner chamber of the eye.
In hummingbirds, the pecten is very large relative to the size of the eye. It takes up nearly one-third of the diameter of the eyeball. This is much larger proportional to eye size than the pecten of other birds. For example, in chickens, the pecten is just a tiny flap, while in hummingbirds it is a prominent structural feature.
Functions of the pecten
The pecten performs several important functions in the hummingbird eye:
Nutrient provider
The abundant blood vessels in the pecten provide oxygen and nutrients to the various parts of the eye that lack their own blood supply. This includes the lens, cornea, and retina. The large surface area of the folded pecten allows for more blood vessels to transfer nourishment to starved eye tissues.
Waste removal
The pecten also removes waste products like carbon dioxide and lactic acid from the eye. Its blood vessels absorb and carry away metabolic wastes that could cloud vision if allowed to accumulate.
Vision enhancement
One of the most important functions of the pecten is improving visual acuity. The rapid fluttering of hummingbird wings can make vision blurry. The pecten acts like a shock absorber, damping out retinal disturbances caused by flight and movement. This allows hummingbirds to maintain excellent visual stability and focus.
The pecten may also enhance hummingbird color vision. It contains high levels of carotenoid pigments that may act as a filter to protect the retina from ultraviolet light and sharpen color perception. Many experts think the pecten plays a key role in hummingbirds’ ability to see red nectar-bearing flowers.
Stabilizing the inner eye
In addition to visual stabilization, the large pecten provides structural support within the fluid-filled hummingbird eye. The inner chamber of the eye does not maintain its own shape without assistance. By spanning the vitreous humor, the rigid pecten helps keep the inner eye from collapsing in on itself. This maintains the ideal spherical shape for optimal refractive power.
Why do hummingbirds need such a large pecten?
Most bird species have a pecten in their eye, but it is disproportionately large in hummingbirds compared to relative eye size. There are a few key reasons hummingbirds require such an oversized pecten:
– High metabolism – A hummingbird’s metabolic rate is incredibly fast. At rest, their metabolism is 10 times higher than humans. In flight, it can reach up to 75 times faster. This creates a huge demand for oxygen and nutrients to power energetic hovering flight. The enlarged pecten with its dense network of blood vessels is designed to meet these extreme metabolic needs.
– Nectar diet – Hummingbirds get most of their calories from drinking sugary nectar. Nectar contains minimal fat or protein. The pecten provides key amino acids, vitamins, and antioxidants hummingbirds don’t get enough of from nectar alone.
– Rapid heart rate – Even while perching, a hummingbird’s heart beats around 250-500 times per minute. During flight, this accelerates to a blistering 1,200 beats per minute. This pumping speed circulates oxygen-rich blood through the pecten’s vessels to satisfy their intense metabolic demands.
– High-speed vision – Hummingbirds exhibit incredibly fast vision to match their aerial agility. Their eyes can process up to 200 separate images per second compared to humans’ 100-150 images per second. The enlarged pecten nourishes the retina and enhances visual clarity for rapid information processing.
– Small size – Hummingbirds are the smallest birds, which places limitations on total blood volume for meeting nutritional needs. The outsized pecten allows a higher concentration of blood vessels in a compact space within the tiny eyeball.
Pecten differences across hummingbird species
While all hummingbird species have an enlarged pecten, there are some variations across types:
Size
The pecten size tends to scale with overall body size. Larger hummingbird species like giant hummingbirds have a proportionately bigger pecten than tiny bee hummingbirds. But in all cases, it takes up a larger proportional space in the eye than other birds.
Number
Most hummingbird species have a single pecten in each eye. But some, like Allen’s hummingbird and the bee hummingbird, actually have two pectens per eye – a lower larger one and an upper smaller one. Scientists think the extra pecten provides added nourishment for these smallest hummingbirds.
Shape
The pecten generally forms a crescent moon shape across species. But there are some variations. Mountain woodstars have a more triangular pecten, while purple-crowned woodstars have a more rounded fan-like shape. Shape differences likely relate to slight variances in eye anatomy across hummingbird groups.
Pigmentation
Many hummingbirds have a tan-pigmented pecten that matches the hue of the bird plumage. But some species, like blue-throated hummingbirds, have a dark gray to black-pigmented pecten. The greater melanin in these pectens may help filter light entering the eye.
Blood vessels
The density and pattern of blood vessels can also differ between types of hummingbirds. For example, green-fronted hummingbirds have a pecten densely packed with tiny parallel blood vessels, while giant hummingbirds have larger blood vessels in a looser arrangement.
Hummingbird Species | Pecten Size | Pecten Number | Pecten Shape | Pecten Color |
---|---|---|---|---|
Allen’s Hummingbird | Small | Double | Crescent | Tan |
Bee Hummingbird | Very small | Double | Crescent | Tan |
Blue-throated Hummingbird | Medium | Single | Crescent | Dark gray |
Conclusion
The prominent white spot seen behind a hummingbird’s eyes is an important anatomical structure called the pecten. This crescent-shaped organ is full of blood vessels that provide nourishment, waste removal, visual stability, and structural support within the hummingbird eye.
Hummingbirds have the largest relative pecten size compared to overall eye size of any bird group. Their highly energetic lifestyle and unique visual needs require the oversized pecten to support their demanding metabolism, provide nutrients lacking in their liquid diet, enable rapid heartbeats to fuel flight, and enhance visual acuity.
Subtle differences in pecten size, shape, pigmentation, and vasculature exist between the over 300 different hummingbird species. But in all types, the pecten plays an essential role sustaining the hummingbird’s signature hovering flight, high-speed metabolism, and visual keenness. The next time you see a hummingbird hover up close, take a peek at its face and marvel at this special anatomical adaptation.