A tropical bird’s eye rings swell when they are in love, reveals new research.
Both male and female Java sparrows display enlarged eye rings to signal their readiness to breed, say scientists.
The findings, published in the journal PLOS One, show that eye ring changes, in addition to duet dancing and vocal and non-vocal sounds, play an important role in communication between courting couples.
Many species of birds are known for their elaborate courtship rituals and romantic gestures that include song, dance, preening, and flamboyant plumage.
But while changes in colorful external attributes during that period has attracted plenty of attention, the role of facial features has not been studied in so much detail.
Professor Masayo Soma and her research group at Hokkaido University in Japan saw increased swelling in Java sparrows’ eye rings – an area of blushed bare skin around the pupils – when they bond with a mate of their choice.
Observed in both females and males, the team say the change acts as a “signal” for mating readiness.
Prof. Soma said: “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.
“In Java sparrows, 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.”
She said Java sparrows are known to be socially monogamous, showing mutual courtship and pair-bonding for a long time.
The experiment took place over 12 weeks and involved 44 adult sparrows from a laboratory population.
Prof Soma said: “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.”
The researchers observed a “significant” increase in the eye rings of both sexes amongst pair-bonded partners throughout the experiment, a change not seen when they were kept alone or with a non-preferred partner.
They explained that with bare skin having the potential to change more dynamically than plumage, which requires time for molting, swollen eye rings act as a rapid indicator of mating readiness.
Prof. Soma added: “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.”
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