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Java sparrows get swollen eye rings when they're ready to mate

In the world of birds, courtship often involves captivating dances, melodious songs, intricate gifts, and dazzling feathers. However, recent research has focused on a less-explored aspect of avian romance – the role of facial features.

Unique phenomenon 

A study led by Professor Masayo Soma at Hokkaido University reveals that in pair-bonded Java sparrows, the enlargement of their eye rings signifies their readiness for breeding. 

“Breeding-related blushing in primates is well studied, as they show conspicuous changes. For example, in rhesus macaque, males with redder faces appear more attractive to females. Other primates, like mandrills, use it to assert dominance. Birds also display colorful bare areas, like beaks and legs, but blood-flow based blush coloring in birds has gone largely unnoticed,” explained Professor Soma.

“In Java sparrows (Lonchura oryzivora), both sexes have bright pink bare skin around their eyes that swell when the birds are in breeding condition. We predicted that changes in eye rings would reflect physiological conditions and signal fertility, especially among mating pairs.”

Social monogamy 

Java sparrows are recognized for their social monogamy, marked by mutual courtship and long-lasting bonds. 

With this in mind, the researchers theorized that the eye ring transformations could potentially mirror their physiological states – signaling fertility, especially among mating pairs.

Focus of the study 

Spanning 12 weeks, the study was focused on 44 mature sparrows from a laboratory populace. Looking for alterations in eye ring size, the team compared bird mating pairs with other sparrows. 

“We compared individual changes in the eye ring size between birds paired with preferred mates, those paired with non-preferred opposite-sex individuals, and single birds,” said Professor Soma.

Mating preparedness 

The conclusions were revealing: a marked upsurge in the eye rings was discernible among the paired partners. This transformation was conspicuously absent in solitary birds or those coupled with non-preferred partners.

The swollen eye rings, being more dynamic than plumage, can swiftly signify mating preparedness. 

“Java sparrows are native to the tropics and breed most of the year. Morphological changes signaling fertility are important for ensuring reproductive synchrony – especially in the tropics, where seasonal cues are absent,” noted Professor Soma.

More about Java sparrows 

Java sparrows, known scientifically as Lonchura oryzivora, are small birds originally native to the islands of Indonesia – especially Java, Bali, and Sumatra. Over time, they have been introduced to various other parts of the world. 


Java sparrows are easily recognizable due to their bright pink beaks and eye rings, contrasting with their grey upperparts, white cheeks, and black head-throat. Their size typically ranges from 15 to 17 centimeters in length.


In their native range, Java sparrows inhabit open grasslands, tree-studded marshes, and cultivated areas, especially where rice is grown, which leads to their alternative name, the Java Rice Bird.


Primarily seed-eaters, Java sparrows have a particular fondness for rice, which unfortunately often results in them being seen as pests by farmers in areas where rice is cultivated. Their diet in captivity can include millet, sprouted seeds, and various fruits and vegetables.


These birds are monogamous, forming strong pair bonds. In the wild, they often nest in tree hollows or building crevices, laying a clutch of 4-8 eggs. In captivity, they can be encouraged to breed using nest boxes.


Java sparrows are social birds, often seen in flocks, especially outside the breeding season. They have a pleasant, bubbly song, which is one reason they’ve been popular in the pet trade.

Status and conservation

The wild population of Java sparrows has declined in recent decades due to habitat loss, hunting, and the pet trade. 

The study is published in the journal PLoS ONE

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