For the souimangas, everything that sparkles is hot.
The bright, iridescent feathers of birds heat up more than other types of feathers, which may make it more difficult to stay cool in hot, sunny conditions. If that is the case, colorful plumes can be expensive adornments to attract mates, the researchers report on August 4 in the Journal of the Royal Society Interface.
Many studies on the thermal properties of animal color have previously focused on the differences between how dark and light colors derived from pigments react to light and heat, says Svana Rogalla, a biophysicist at the Biofisika Institute of Bilbao, Spain. Less attention has been paid to how heat interacts with structural color – often vibrant, iridescent hues created by the refraction of light on microscopic structures. These structures are found in everything from scales to deep-sea worms (SN: 05/25/2020) spider exoskeletons (SN: 9/9/16).
In the new study, Rogalla and his colleagues turned to souimangas, nectar-feeding birds that are typically smaller than sparrows from Africa, Asia and Australia. Souimangas have feather colors that come from a mixture of sources, including pigments like fiery carotenoids and dark melanin as well as an iridescent structural color.
The researchers examined 15 species of sun-palm trees kept at the Field Museum in Chicago, heating the specimens under a lamp that mimics sunlight, then measuring the surface temperatures of the feathers and skin of the birds kept.
The team found that the iridescent feathers heated up more than the feathers which only had pigment-based colors. In breast feathers, for example, the iridescent feathers averaged around 74 ° Celsius on the surface of the feather after a few minutes of light exposure, where feathers with yellow to red carotenoid pigments reached a just over 63 ° C. This heat also transferred to the skin, with skin under iridescent feathers becoming about 5 degrees C and 8 degrees C warmer than skin under feathers with olive or yellow to red pigments .
Scattering light from microscopic layers of air, structures containing keratin and melanin called melanosomes create the glow of iridescent feathers. But the abundant melanin also causes the feathers to absorb more solar radiation than other lighter-colored, less melanin-rich feathers, Rogalla says, which helps with heating.
In addition, high power microscopes and thermal simulations have shown that the arrangement of melanosomes in an iridescent feather promotes heat absorption. Melanosomes in a dark, non-iridescent feather are relatively disorganized in layers. But the melanosomes in the microscopic stacked structures that produce the iridescence are more organized and densely packed. Iridescent feathers probably heat up more than black feathers, as these shimmering plumes have more melanosomes over a given feather area, says Rogalla.
The shape of melanosomes can also make a difference. Iridescent feathers tend to have plate-shaped melanosomes, with black feather rod-shaped melanosomes and gray feather spheres. The team’s simulations showed that the plaque-shaped melanosomes heated up more than the other two.
Iridescence is more pronounced in male sunbirds than in females and is used in courtship displays. Rogalla wonders if flashy colors, possibly contributing to heat stress, create an evolutionary conflict between mating and staying cool, or thermoregulating. “It could be a compromise,” she says, “being very colorful to attract females, but then you have a cost when you sit in the sun when it’s hot.”
Allison Shultz, an ornithologist at the Los Angeles County Natural History Museums in California who was not involved in this research, says the vivid plumage can serve as a signal demonstrating the relative vigor and resilience of a male bird. “Maybe it shows [females] that men are able to cope with this higher heat load by having iridescent colors.
Still, there is a big caveat to the experimental setup. The results of the study can be “difficult to put into perspective” because the experiments were carried out on dead birds, says Vinod Kumar Saranathan, evolutionary biologist at Yale-NUS College in Singapore. “You don’t have the advantage of the living bird [to evaluate], manipulating his body and doing various things to actively thermoregulate himself.
Souimangas can release heat by panting or changing the position of the feathers. And birds rarely stay in the sun for long, explains Saranathan, explaining that they switch flowers from sunny areas to more shaded forest. If the iridescent feathers lose heat as quickly as they acquire it, the souimangas might use the shade to cool off, mitigating the effect of the warm plumage.
“Iridescence has evolved so many times independently through the [bird evolutionary tree], says Saranathan. If the shiny feathers were particularly distressing for birds, he says, this repeated development would not be likely.
Even so, the findings have implications for how iridescence might contribute to the heat effects of climate change. Rogalla points out that small birds like souimangas – vulnerable to large fluctuations in ambient temperature – have already perished en masse due to episodes of extreme heat. Such cases of massive death have occurred among Australian parrots, for example, where intense heat waves have killed hundreds of parakeets and dozens of cockatoo in recent years. With their warm feathers, iridescent sunbirds can be particularly at risk.
A warming climate may force iridescent birds to be active for shorter periods of time on hot days to avoid the additive impact of solar heating, says Saranathan. If so, he adds, such a truncated schedule can increase competition for food in the precious remaining working hours.