Vision is one sense where birds beat humans. While we often struggle to read roadside signs whilst traveling at 100 km h−1, falcons flying at the same speed can locate rodents over 2 km away. In fact, all kinds of birds rely on sight for in-flight navigation and balance. Birds have large eyes that are densely packed with specialized light-detecting cells at the back of the eye, in the retina. Most animals – including humans – have blood vessels in their retina, which deliver oxygen to the light-detecting cells. Mitochondria within the cells are responsible for using oxygen to make energy, powering the cell’s activity. However, birds lack these blood vessels in the retina, leaving scientists puzzled as to how birds power their spectacular vision. Without blood vessels in the retina, birds rely on high oxygen levels in the air to increase the oxygen supply of blood within their arteries. Oxygen from arterial blood fuels mitochondria in the eyes’ light-detecting cells, but when the blood oxygen supply is low, it becomes difficult for oxygen to reach the mitochondria. When birds fly and ascend to high altitudes, they experience a challenge like that of a mountain climber nearing the peak. The atmosphere gets thinner at higher altitudes, resulting in less oxygen within the air. Naja Christensen, Kristian Beedholm and Christian Damsgaard from Aarhus University, Denmark, speculated that low oxygen makes it harder for birds to see.
To find out, Christensen and the team assessed how low oxygen levels affect vision in zebra finches (Taeniopygia guttata). The researchers placed each bird in a transparent plastic cylinder, surrounded by a black and white drum that could rotate. When the drum was rotated, the birds could see the movement of the black lines on the drum wall. When birds detect movement, they instinctively turn their head toward the action. Watching for this reflex allowed the research team to assess whether the birds were able to see movement. So, Christensen and colleagues videoed the birds whilst rotating the drum at increasing speeds until the bird could no longer perceive the movement. The quicker the rotation that a bird responded to, the better the bird's vision. The team tested the vision of eight finches at ‘normal’ oxygen levels (21%, as would occur at sea level) and in low oxygen conditions (10%, the effective oxygen percentage experienced by bar-headed geese 5000 m above sea level). If the birds detected movement better at 21% oxygen than at 10% oxygen, it would indicate their vision is likely worse during flight.
Sure enough, all of the birds were able to detect motion at faster rotation speeds when tested at 21% oxygen. More impressively, the birds still reacted to the movement of the spinning drum at the fastest speeds in just 10% oxygen. They could still see, but they responded 25% less, suggesting that their vision was not as reliable. The team was surprised that the birds’ ability to perceive movement was not lost entirely at low oxygen levels. The fact that the birds still maintained the ability to follow movement at low oxygen suggested that the eyes’ light-detecting cells were functioning despite losing their oxygen supply. But how?
It turns out birds’ eyes might not be entirely fuelled by oxygen. Rather, the light-detecting cells in the eyes may be powered anaerobically, which requires enormous quantities of glucose to provide energy. This possibility needs more research, but Christensen and the team propose that glucose power is likely because birds have special compartments in their light-detecting cells that may store glucose, giving them a reliable supply of the sugar to fuel the mitochondria during flight, allowing them to see.
Although it has long been known that birds have specialized eyes to allow acute vision during flight, the discovery that birds’ eyes might operate on an entirely different fuel source to other animals is remarkable. If only human eyes worked this way so that our vision could be improved by our favourite candy bar.
