Iridescence in Hummingbirds

Iridescence, a striking hallmark of hummingbirds, is a remarkable feather colouration actually produced by structure rather than pigment. The appearance of colour in birds is created by either pigment or feather structure, and sometimes a combination of the two. Iridescence is actually produced by the reflection of microscopic structural features on the feather surface. This dazzling display of shimmering colour that at different angles becomes a complete absence of colour is one of the signature characteristics of hummingbirds. How is it produced and what is its purpose?

Rufous Hummingbird

Anna's Hummingbird

The most common example of iridescent colouration is the changeable rainbow colouration seen in oily puddles on pavement. How do the layers of oil and water create all these colours? Here is a somewhat technical explanation of how this works: As light reaches the surface of the oil, some light is immediately reflected from the surface and some light passes into the oil. The light that passes into the oil slows down and becomes compressed because the oil has a higher density than air. Some of this light then reflects off the bottom of the oil layer and then moves back to the surface. Then at the surface this light from the bottom meets the light reflected from the surface. These two light waves can be “out of phase” (the peaks and troughs do not match up), meaning they cancel each other out; however some wavelengths will be in synch and can be amplified, displaying a variety of colours. This process of cancellation and amplification results in a brilliant colour display which changes according to the angle of observation, i.e. how we are positioned in relation to the oil slick as well as the angle of the sun as it hits the surface.

Iridescent feathers in birds produce their colours in a similar fashion to oil slicks. Below is a diagram which shows how sunlight plus angle of observation plus light wavelengths interact to produce vivid, dull or black colour. Light hitting the feathers may bounce back from any one of the layers and it adds together with light bouncing back from other layers. If the light waves add up, the observer sees bright, flashing colour. If they cancel each other out, the feathers appear black. If they are slightly out of synch, then one sees a duller colour.

  

The feather structures which create iridescent colours in hummingbirds are very complex, with multiple layers of air bubbles. In typical hummingbird iridescent colouration there are eight to ten layers of feathers tightly stacked on top of one another. Amazingly, it is the combined reflections from inner and outer surfaces of the air bubbles in the feather layers that create iridescent colours. This arrangement scatters and refracts light at a microscopic level and gives the feathers their shifting iridescence. The entire system is incredibly precise and uniform, creating what appears to be a consistent iridescent colour. This feather stacking will intensify and purify the resulting spectral colour, which explains why hummingbirds have possibly the most intensively iridescent feathers known in birds.

Black-chinned Hummingbird

Calliope Hummingbird

In hummingbirds these feather layers are arranged at a very specific angle so that the bounce-back of colour can be directional and targeted. This mechanism allows the hummingbird to give precise, ‘directional’ signals. The males, which are intensely iridescent, can deliberately signal rivals and attract females, while remain unobvious to predators. Females also have some iridescent feathers on their throats, which they use along with their tail feathers, to protect food resources in their area from other hummingbirds. This is particularly evident when they have a nest with young ones. It is believed that females may produce more iridescent throat feathers as they mature.

It is little wonder that hummingbirds are considered one of the most fascinating species of the avian world with their brilliant yet ephemeral iridescence, which is actually no ‘colour’ at all!

Anna's Hummingbird