Java sparrow

The Java sparrow is about 15 to 17 cm (5.9 to 6.7 in) in length from the beak to its tip of tail feathers. Although only about the size of a house sparrow, it may be the largest species in the estrildid family. The mean body mass is 24.5 g (0.86 oz), making it slightly heavier than its nearest known rival, the black-bellied seedeater. The adult is unmistakable, with its grey upperparts and breast, pink belly, white-cheeked black head, red eye-ring, pink feet and thick red bill.

Both sexes are similar. Immature birds have brown upperparts and pale brown underparts, and a plain head. Very young birds have a black beak with a pink base.

The call is a chip, and the song is a rapid series of call notes chipchipchipchipchipchip.

Java sparrows produce distinct trill-calls in different behavioral contexts, according to a study by Furutani et al. (2018). These trill-calls, though acoustically similar, vary based on their repetition rate, which changes depending on the situation. In aggressive interactions, the sparrows emit faster trill-calls with higher sound pressure levels and entropy. In contrast, during affiliative behaviors, the trill-calls are slower and softer. This variation in trill-calls plays a crucial role in the birds' social communication, helping them convey different intentions based on the context.