Guest Author: Cynthia Berger
IN THE EARLY 1970s, A RESEARCHER testing the ability of pigeons to discriminate colors discovered by accident that the birds can see ultraviolet (UV) light. The finding was deemed curious but not too important. “It was natural for scientists to assume that bird vision is like human vision,” says Geoffrey Hill, an Auburn University ornithologist and the author of Bird Coloration. “After all, birds and humans are both active by day, we use bright colors as cues. … No one really imagined birds might see the world differently.”
But during the following decades, systematic testing of bird vision revealed something unexpected: Many bird species—not just pigeons—can see UV light. Indeed, with the exception of night-flying birds such as owls, the eyes of most birds probably are even more sensitive to ultraviolet light than they are to what we call visible light. Scientists also have learned that many birds have plumage that reflects UV light. Together, these discoveries “made us realize there could be new answers to old questions,” says Drake University biologist Muir Eaton. Birds rely on vision to choose mates, find food and scan for predators, for example. “If you assume birds see exactly what we see, you could have the wrong framework for understanding bird behavior,” Eaton says.
Consider how birds choose mates. “After the first studies on birds and UV came out, people started saying, ‘Maybe your study of mate choice isn’t valid because you scored the feather colors with the naked eye,’” says Peter Dunn, a University of Wisconsin–Milwaukee biologist who studies active little warblers called common yellowthroats (below). Adds Hill, who has researched mate choice in house finches, bluebirds and indigo buntings: “When I started working, back in the 1980s, we used to hold up color charts against the birds’ feathers”—the same square paint chips that are an industry standard for graphic designers and interior decorators.
During the past three decades, a flurry of studies has tested the intriguing notion that mate choice and other bird behaviors may be shaped by secret visual signals humans cannot see. Though the premise was exotic, what facilitated this explosion of research was prosaic: Technology got better and cheaper. In particular, the increased availability and decreased cost of a lab device called the spectrophotometer—which precisely measures light reflected or absorbed by a surface—let scientists, if not see like a bird, at least quantify what birds are seeing.
Initially, many researchers turned their spectrophotometers on birds that do not use flashy feathers to attract mates. A team of Swedish scientists, for example, looked at the blue tit, a European relative of the chickadee. As with many bird species, male and female blue tits look alike to humans. “Standard literature describes the plumage as closely similar between the sexes,” says Staffan Andersson, a professor of animal ecology at the University of Gothenburg. “The main problem with this conclusion is that it is based on the UV-blind and yellow-biased human eye.” Using a spectrophotometry probe to scan the feathers of wild-caught birds, Andersson and his colleagues discovered that blue tits themselves have no problem telling males from females: Males have a patch of feathers on the crown of the head that strongly reflects UV light; females do not.
Blue tits are not alone. In 2005, Eaton used a spectrophotometer to scan the plumage of museum study skins of 139 songbird species in which males and females appear alike, from cedar waxwings to barn swallows to mockingbirds to western meadowlarks. Though scientists previously had classified these birds, along with 70 percent of all songbird species, as sexually monochromatic (males and females looking identical), a full 90 percent of the species Eaton scanned actually were sexually dichromatic: different once you took into account the better discrimination of colors (including ultraviolet) by birds and the amount of UV light feathers reflect. “To the birds themselves, males and females look quite different from one another,” Eaton says.
Such findings led some researchers to speculate that the primary role of avian UV vision is to select mates. Indeed, in laboratory tests, Andersson and his colleagues found that female blue tits strongly preferred males with the brightest “invisible” crowns—evidence that the UV-reflecting feathers humans cannot see were serving their function.
Over time, however, scientists have concluded that blue tits are the exception to the rule. Very few bird species use UV light only—with no other visual cues—to attract and choose mates. “In general, ultraviolet reflectance simply reinforces the plumage color patterns we humans already can see,” says Dunn. Among his study subjects, “yellowthroat females do prefer males that are brighter, but not because of the UV reflectance alone. It’s more the brightness of the feathers overall.”
Foiling Nest Parasites
So, how do birds use their power of UV vision? In a surprising number of ways, scientists propose. Many songbirds, for example, are pestered by nest parasites: birds such as cuckoos and brown-headed cowbirds that dump their eggs in a host nest and leave the hard work of childcare to the unwilling adoptive parents. It turns out that some potential hosts are able to recognize and reject eggs that, to human eyes, look like their own. Might birds be responding to UV signals rather than to colors visible to people?
The evidence so far is suggestive but inconclusive. In one 2007 study in the Czech Republic, song thrushes rejected experimental eggs researchers had designed as perfect mimics. It turned out the scientists’ eggs had a UV reflectance different from the thrush eggs. But a Canadian study of 11 species parasitized by cowbirds found no correlation: Some species accepted eggs that were a UV match; others rejected them.
Signals From Hungry Chicks
Scientists also are investigating whether UV signals play a role after eggs hatch. Think of hardworking parent birds, ferrying caterpillars to a nestful of hungry chicks. Which chick gets fed first? In some species, parents cue in on a hatchling’s size or how loudly and energetically it begs. But color also is a factor—the brightness of the gape (edge of the mouth) or the head seems to stimulate a parent to proffer food. Some researchers suggest UV color may enhance this effect.
Newly hatched European rollers, for instance, have a patch of bare skin on the foreheads that reflects UV light. Their parents face a particular challenge as they dole out centipedes and other treats: Because roller clutches hatch over a period of days, first-hatched chicks are larger and need more food than chicks that hatch later. In a 2011 study, Spanish researchers noted that heavier chicks tend to have the least UV-reflective forehead patches; lighter chicks had more reflective foreheads. To test whether this difference helps parents decide who to feed the most, the scientists smeared a sunblocklike lotion on the foreheads of some chicks, using a control lotion on others. The chicks with the blocker gained less weight than their unblocked nestmates—clearly showing they got less food when they could not advertise their nutritional status with UV signals.
Parent birds may rely on UV signals when they’re off finding food as well. Many insects, includingmoths and butterflies, have body coatings that strongly reflect UV light. Many seeds also are reflective, and berries and fruits develop a highly reflective waxy coating as they ripen. On the other hand, most green leaves do not reflect UV light. So even if a red berry seems quite visible against a green leaf to human eyes, for birds this contrast is enhanced.
“I think the biggest thing to come from the discovery that birds see in the ultraviolet is our understanding of how some predatory birds find their prey,” says Hill. Picture, for example, a kestrel (American kestrel, right) perched high on a telephone wire, surveying a field far below. “I always wondered how a bird of prey gets enough to eat,” he says. “After all, you can walk through a grassy field 20 times and never see a mouse.”
But that’s because we do not see what the birds see. It turns out that one key prey for common kestrels, the meadow vole, behaves like a tiny dog, using squirts of urine to mark its trails through tall grass. About 15 years ago, Finnish researchers from the University of Turku discovered that vole urine reflects UV light—which kestrels soaring over open fields can plainly see. “Once you realize raptors can follow the trail right to the animal, it makes a lot more sense,” Hill says.
Indeed it does. While people long have wondered what it would be like to soar like a bird, the more interesting question—particularly for biologists—may be: What would it be like to see like a bird?