Yifeng Shi

Hummingbird's Red Neck:  Structure Decides Color

One of the most common birds here on UC Berkeley campus is Anna's hummingbird. Hummingbirds, overall, are known for their small size (~ 4 inches, or 10 cm) , extremely fast wingbeat (up to 200 times/second) , and the long beak & tongue for getting nectars from flowers. If you are, by any chance, around Berkeley campus, there is a high chance that you will catch their unique buzzing sound before spotting these tiny agile creatures. 
Despite their being so common, I barely noticed their existence before I started birdwatching. Once I started to grow awareness of the surroundings, they pop up in front of my eyes literally everyday. Over the past half a year, holding a tiny camera here and there, I got many good chances to record these tiny birds moving around, and that was a brand new experience for me.  ↓

One thing I noticed not long after I started birdwatching was that for male Anna's humming birds, they all have a black patch on their necks. These dark feathers on the neck can turn into shining iridescent red or rose-pink color when they tilted their heads at a specific angle. Have you ever watched face changing in Sichuan opera?  Seeing these birds shaking head back and forth reminds me of this type of show I watched years ago.  The fact that male (Anna's) hummingbird change the color of the plumage on the neck is a well-known fact to many birdwatchers. But to me at that moment, it was a huge discovery and I stopped by the same tree every day to try my luck catching a nice photo/video of their face changing. ↓ 

-------------------------------------------

Most of the colors we see in our everyday lives are from pigments or dyes. Those colors do not  change with the direction of the incident light. Common examples of pigments including the green color in leaves (chlorophyll) and the red/orange colors in some sandstones (iron oxide, Fe2O3). Other than plants and minerals, living microbes can also display a wide range of colors by producing different types of pigments. If you have ever been to the Yellowstone national park, the colors displayed at Grand Prismatic Spring are mostly from microbes that generates different types of pigments. These color displays are not affected by the angle of the incident light, as they their working principle is solely based on the chemical structures of the pigment: certain pigment absorbs  specific wavelength of light and reflect others back into our eyes. This process is only dependent on the molecular structure of the pigment itself. Take chlorophyll as an example again, it absorbs blue and red light, therefore displaying green color. 

This absorb-and-reflect mechanism explains a large number of colors we see in our everyday lives. However, the light-matter interaction is more complicated than absorption and reflection.  They also transmit, scatter, interfere, diffract, etc. Therefore, even without the presence of pigment, one might still see color display through other mechanisms from light-matter interactions. In the context of hummingbird's red neck, light interference is the main reason of this stunning color change. To make the interference happen, hummingbirds developed very unique microstructures in their neck feathers.  I unfortunately was not able to get a sample of their neck feather, but found a nice research article revealing this secret in detail. Check the figures below and read the text underneath from left right to understand the structure of the feathers with length scales getting smaller and smaller. All three figures in the panel below are adopted from J Comp Physiol A 204, 965–975 (2018). 

↑  The center shaft of a feather is called rachis. Along rachis,  "secondary shafts" come out, which is called barbs. The small fibers extended along barbs are called barbules, and it's the microstructure of barbule that makes the color difference. 

↑  (a) Shows the SEM image of barbules along a barb.  (b) displays the similar structure but from a different angle (follow the arrow, arrowhead, and asterisk!)  When further magnified and cut a barbule, the cross section is made of layered structures as shown in (d). 

↑  Further magnifying the cross section of barbules, one can see it is made of multiple layers of stacked melanosome.  These melanosomes are layered with keratin and air gaps in a regular way. More importantly, the gap are at the scale of the wavelength of visible light. It creates a perfect platform for light interference. 

→   Thin film interference. How a layered structure ended up in stunning color change? When incident light hits the barbule, it has multiple pathways to go through (the graph is over-simplified just for easy-to-understand) :   

1) Hitting the uppermost layer of melanosome, and reflect 
2) Transmitting through the first layer of melanosome, and reflect at the second layer
3) Transmitting through the first two layers of melanosome, and reflect at the third layer
..... so on so forth.

The reflected waves combine and form constructive interference. The specific gap size in Anna's hummingbird neck feather favors the constructive interference at wavelength between 650-700 nm. In simple words, red light gets enhanced by these tiny gaps, and you see more red color as a result. 
This mechanism is pretty common in our everyday lives -- the rainbow color from a soap bubble is also the product of thin film interference. But for soap bubble does not have a fixed thickness of the film, therefore allowing different wavelengths of light to be constructively interfered.  for Anna's hummingbird, the layer gaps are relatively uniform, thus only enhances red color. 

←  Nothing is perfect, however, including hummingbirds. Within the barbules, each layer of melanosome has its own thickness, and the gap size can never maintain exactly the same. Slight variation in the gap distance can produce a different way of interference and thus a different color in our eyes. Although the majority of the male hummingbirds I see have the all-red-head, every once in a while I do observe colors of orange, yellow, or even green on their necks from specific angles. I caught the photo on the left from a very cooperative hummingbird constantly shaking his head back and forth. Here you can see a variety of colors displayed instead of all-or-nothing. 

The last fun fact I learned recently about Anna's hummingbird is that this color-changing features are not limited to males.  Female birds can have similar plumage structures on their necks, but the number of the feathers is very limited and only located at specific area (center of the neck). Their color-changing feathers also have less regular structures, therefore might reflecting colors with a wider variety other than  rose-pink/red.  Generally, it is hard to tell a bird if it is an "adult female" or a "juvenile male". This question confused me a lot. There are quite a few suspicious candidates in my albums that i put them on the right. If you have a good way to distinguish them, please do let me know!  → 

There are, of course, too many things that I haven't figured out at this moment, including the question I have for all living species in this world: why they evolved in such a unique way with this degree of fine-tuned details? Not just hummingbird, this thin-film reflection feature is observed in many other birds including mallards, peacocks, and even pigeons. Their color-changing plumages share similar features with hummingbirds but are unique in their own ways (e.g., shape of the melanosome of mallard is rod-like, not plate-like for hummingbirds). Digging further into these topics can be an endless yet fun exploration. 
Colors, or more broadly, the different ways light interact with materials are always intriguing topics to me.  During my work in the wet chemistry laboratories, colors are important standards I use to evaluate the quality of my samples. Outside work, they inspire my curiosity about the nature and allow me to appreciate the beauty of the world. 

Last updated: January 2026