The head pattern does most of the work
A lost Java sparrow is picked out by its head before anything else. The classic bird has a jet-black head and throat, large clean white cheek patches, and a surprisingly thick, conical pink bill, set over a smooth dove-grey back and a white belly. That black-white-and-pink head looks almost painted on and separates a Java sparrow at once from house sparrows, zebra finches and other small seed-eaters a finder might assume it is. Tell rescuers the bird is finch-sized and plump, not sparrow-slim, and that the pink bill is the feature to confirm in any photo, even a phone shot taken through a window.
Say the colour form, because not every Java sparrow is grey
Pet Java sparrows are also kept in white, pied, silver and fawn forms, and a finder briefed only on the grey bird will not connect a pure white one to your alert. State your bird's form first, then the marks that remain constant: the thick bill shape, the white cheek on coloured birds, the dark eye with its fine ring, and any leg band colour. Note whether your sparrow lived with a mate, because a bonded Java sparrow may call persistently and stay near where it was last seen, and mention its song — a musical, bell-like series quite unlike a house sparrow's chirp — since a finder may hear a caged-sounding song coming from a hedge.
How a finder should approach a bird this small
A Java sparrow will not come to a hand the way a tame parrot might, so finder guidance matters. Suggest watching where the bird feeds, putting seed and shallow water in a quiet spot, and placing an open small cage nearby with seed inside, because a cage-bred finch often treats a cage as shelter rather than a trap. Ask finders not to lunge or throw cloth over open ground, which scatters a bird this light into the wind, and to photograph it perched if capture is not possible, noting the street, time and direction it flew. If a rescue takes in a Java sparrow, the bill and cheek pattern plus your band note should be checked before it is grouped with other finches. Also note the cage it escaped from and the seed mix it was feeding on, because a Java sparrow that finds familiar seed in a quiet garden is far more likely to stay in one place long enough for a finder to photograph the bill and cheeks clearly.
One close photo beats a folder of distant ones
For a species whose members look deliberately alike, identification lives in one sharp close portrait. Keep a recent head-and-shoulder photo in soft daylight, a note of the colour form and band, and your contact details together and ready to send, and refresh the photo if your bird is a youngster whose bill colour is still settling to full pink. A finder comparing a handed-in finch needs that portrait at the moment of comparison, not after the bird has been moved on. With that portrait and those details held on a KinsLe Pet Pass, a rescue can confirm a Java sparrow is yours from its black head and pink bill in a single look.