Java Sparrows Use Eye Rings To Communicate Their Breeding Condition

Java sparrows are small, colorful birds that are native to Southeast Asia and have been introduced to many other regions of the world.

They are popular as pets and are known for their elaborate courtship rituals, which include singing, dancing, and exchanging gifts. But how can you tell when these birds are ready to mate?

A new study by researchers from Hokkaido University in Japan found one of the clues in their eyes.

Eye rings swell when pair-bonded

(Photo : ADEM ALTAN/AFP via Getty Images)

The study, published in the journal PLOS ONE, reported that Java sparrows show increased swelling in their eye rings--an area of blushed bare skin around the eyes--when they bond with a mate of their choice.

This change was observed in both females and males, and it acted as a signal for mating readiness.

The researchers conducted an experiment with 44 adult Java sparrows from a laboratory population.

They compared the eye ring size changes between birds paired with preferred mates, those paired with non-preferred opposite-sex individuals, and single birds.

The experiment lasted 12 weeks, during which the researchers measured the eye ring size of each bird every week.

They found that the eye rings of both sexes among pair-bonded partners showed a significant increase throughout the experiment, reaching a maximum size after about eight weeks.

This change was not seen when the birds were kept alone or with a non-preferred partner.

The researchers speculated that the swelling of the eye rings was caused by increased blood flow to the area, which could be influenced by hormonal changes related to breeding.

They also suggested that the eye rings could serve as a rapid indicator of mating readiness, as they could change more dynamically than plumage, which requires time for molting.

Also Read: Male Sparrows Wave Wings Before Engaging in Brawl

Eye rings help synchronize breeding in the tropics

The researchers pointed out that the eye ring changes could have important implications for the reproductive success of Java sparrows, especially in their native tropical habitat.

Java sparrows are socially monogamous, meaning that they form long-lasting pair bonds with their mates. They also breed most of the year, unlike many other birds that have distinct breeding seasons.

According to Associate Professor Masayo Soma, who led the study, Java sparrows need morphological changes signaling fertility to ensure reproductive synchrony in the tropics, where seasonal cues are absent.

She also said that Java sparrows are one of the few bird species that display colorful bare areas, like beaks and legs, but blood-flow-based blush coloring in birds has gone largely unnoticed.

The researchers also noted that the eye ring changes could be part of a complex communication system between Java sparrows, which also involves vocal and non-vocal sounds, duet dancing, and gift-giving behaviors.

They suggested that future studies could explore how these different signals interact and influence each other during courtship and mating.

The study is one of the first to investigate the role of facial features in bird communication, especially in relation to breeding.

The researchers hope that their findings will inspire more research on this under-investigated aspect of bird behavior.