Liliana D’Alba of Naturalis Biodiversity Center explores how feather colouration can affect bird’s body temperature and even make flying more efficient. This blog is part of our colourful countdown to the holiday season where we’re celebrating the diversity and beauty of the natural world. Click here to read the rest of the Colour Countdown series.
In nature we constantly see examples of animals that use the colour of their skin, fur or plumage to avoid being seen by predators. For example when a moth seems to disappear into the bark of the tree where it rests. Predators too, use colours to prevent being spotted by their prey, like when a tree python, which is green with white and grey spots, effectively blends with the background vegetation of the tropical rainforest. However, colours not always help animals hide, in fact, in many situations it is very useful to be spotted easily or appear vibrant and ostentatious. In these cases, colours may communicate specific messages to other individuals of the same or different species. A golden poison frog, for instance, parades its bright yellow skin with a warning: “leave me alone, I am poisonous to eat”. In birds, colourful signals produced by males could instead mean: “I am healthy” or “I am a good father”.
But colouration can also have another very important function, it can contribute to the warming or cooling of skin and tissues, helping the animal, on one hand, to avoid exposure to dangerous temperatures, and on the other, to perform certain functions more efficiently. This is because coloured tissues can differentially reflect or absorb solar radiation that can directly affect body temperature. All things being equal, dark colours absorb more solar radiation than light ones. For example, when a turkey vulture perches itself in the sun to warm up on a cold morning, the surface of its black plumage is heated by the sun. This effectively prevents precious body heat from escaping against an otherwise cold plumage surface.
Scientists have researched the thermal consequences of colouration in several groups of ectothermic animals*, that is, organisms that cannot generate their own body heat. In butterflies for instance, researchers have shown that dark wing colours can overheat within seconds when basking in the sun. In these small organisms, the ability to absorb or reflect solar heat could be a matter of life and death, particularly in a warming world.
Birds on the other hand, are endotherms, producing their own body heat and maintaining their internal temperature within a very narrow range. They are also covered in feathers, which are critical to their ability to thermoregulate and show remarkable variation in colour and colour patterns. Plumage clearly has helped birds thrive in diverse environments and colour may play an important part in it. Yet, how coloration affects the way in which heat is gained or lost through plumage, how it influences the metabolic costs of maintaining stable body temperatures or how thermal effects have helped shape the evolution of colourful plumage is not well understood.
Recently, our research team has conducted a series of studies to find answers to these questions. For these studies we have used a combination of techniques including: spectrophotometry – to measure reflectance of differently coloured tissues; infrared videography; experimental flight simulation in a wind tunnel; artificial heating of stuffed bird specimens and comparative analyses.
One of the most interesting evolutionary trade-offs we look at is the way in which colouration can help bird eggs keep warm or avoid overheating. Ground-nesting birds like most ducks, shorebirds and many seabirds should have highly reflective eggshells to avoid overheating because they often get exposed to solar radiation for long periods of time. However, unpigmented eggs are rarely observed in these species, mostly because ground birds also need to produce cryptic eggs to avoid predation. We experimentally heated eggs under controlled illumination to investigate whether the degree of egg heating and cooling depends on how much light is reflected by the eggshells. Importantly, we looked at both the colours of the eggs (the reflectance that lays within the visible part of the light spectrum) and their reflectance in the near infrared (NIR) part of the spectrum (from 700nm to 2000nm). NIR is not often investigated in biological thermal studies, despite the fact that almost half of the solar radiation that reaches the Earth is within this range. We have found that both the colour and the NIR of eggshells strongly determines how much heat is absorbed by the eggs and that species that lay their eggs in environments with intense solar radiation, for example in tropical habitats, tend to produce highly reflective or brighter eggs.
In another set of studies, led by Svana Rogalla, we showed that dark feathers could help birds fly more efficiently. After heating wings with bulbs that simulate the intensity of solar radiation during a sunny day we were able to confirm that dark wings become warmer than light ones, even in windy conditions. Then, the use of the wind tunnel allowed us to simulate flight under realistic conditions and with it we showed that the higher temperature of darker bird wings results in an increase of flight efficiency of up to 20%. Additionally, we studied the evolutionary patterns of wing colouration in seabirds, which often preferentially display dark wings and are very efficient fliers. We found that through evolution wing coloration has changed along with other variables that help increase flight efficiency and minimize the costs of flying, like wingspan.
Another important question we are trying to answer is how heat interacts with structural colour, i.e. that produced without pigments, created by light refracting off of microscopic structures and often characterized by vibrant, iridescent hues such as those of beautiful birds like hummingbirds and sunbirds. Here, the use of electron microscopy and optical and thermal computing simulations are needed to understand how the organisation of structures within an iridescent feather increases heat absorption.
As we gather more evidence about the importance of coloration on the way animals are able to exchange heat with their environment it becomes clear that the future of avian colouration research is interdisciplinary, where different research angles converge to help answer, from the most specific questions about mechanisms, to the broadest about the way in which colours can help animals respond to and adapt to a fast changing world.