Biologists have found a rare rose-breasted Grosbeak which appears genetically female on one side and male on the other. This gorgeous songbird is split right in the middle: with a pink wing pit, pink breast spot on its right side (male plumage) and yellow wing pit on its left (female plumage). The bird’s right wing is longer and has dark feathers compared to the left wing which is smaller and brown in colour. Even the tail feathers are split in colour showing crucial sexual differences.
This condition is called bilateral gynandromorphism. In birds the sex chromosomes- either W or Z determine the resulting chick’s sex- females are ZW while males are ZZ. When a bird produces an egg, it discards half of its chromosomes in a small package called polar body before it gets fertilized. If both- the eggs nucleus and its polar body are fertilised by two different sperm cells, and these nuclei start dividing separately, each side of the offspring will develop its own sex as one side has male chromosomes (ZZ) while the other has female chromosomes (ZW).