Bird Nerd: The eyes have it
YOU MIGHT KNOW of British mathematician Alan Turing, who invented the first programmable computer to crack the German’s secret Enigma code during World War II. But he was also responsible for publishing a mathematical concept in 1952 that explained the hidden order underlying how patterns form in nature, including spots and stripes on animals.
Most birds, in particular, exhibit some degree of patterns and colours. Australia’s diminutive zebra finch, for example, was so named because of the zebra-like black and white bars on its rump and tail. But it also has many other colours and patterns, from a bright orange bill and cheek patches to fine white spotting along its reddishbrown sides.
The zebra finch is Australia’s most abundant and widely distributed grassfinch species, occurring throughout most of mainland Australia. It’s a common and familiar bird in the drier parts of the country, where large numbers congregate around watercourses, their chorus of nasal ‘tiah’ and ‘teh-teh’ calls filling the air.
When we see such a highly patterned bird we assume all individuals of that species have their spots, stripes and blotches in the same places. But look closer and you’ll see that the quantity and design of these patterns varies between individuals. And every now and then a bird exhibits a more obvious plumage variation. Occasionally, we see one that has larger than usual pale areas of plumage or, more rarely, has lost its normal patterning altogether.
Colouration and patterning in all animals is caused by a range of pigments. Melanin is responsible for blacks and browns, and a lack of this pigment can cause a partial or total loss of an individual’s dark patterning. The two main terms that describe these anomalies are albinism and leucism. Both conditions are genetic and inherited and both can lead to a very similar physical appearance.
Leucism, however, causes a lack of the pigment cells that produce melanin. But albinism causes the production of melanin pigment to be reduced or absent. Can we distinguish between the two conditions without the help of a cellular biologist? Yes, and the trick to telling them apart is all in the eyes.
Albino animals have fully unpigmented red eyes. Leucistic animals, on the other hand, never completely lose pigment from the eye, although they can have blue eyes (heterochromia) due to a partial loss of pigment. Heterochromia is most common in animals that are fully leucistic.
Why don’t we see more albino or leucistic birds? Because the lack of melanin reduces the strength and durability of the affected bird’s feathers, making them more prone to breakage. Additionally, the bird’s vision and hearing is negatively affected, making it less able to hunt. The brighter plumage and lack of patterning also makes them easier for predators to see.
Birds can also produce an excess of melanin. This results in the affected bird being darker than usual.