The minimum, and why length is the number that matters
A Bourke's parakeet needs a cage measured first by its longest side. For a pair, treat 24 inches long by 18 inches wide by 18 inches high as the working minimum, and step up to a 30- or 36-inch-long flight cage whenever the room allows, because the difference shows immediately in how much the birds actually fly. Bourke's are Australian grass parakeets that travel in low, level bursts, not vertical climbers that use a tall cage's airspace, so a tall, narrow cage that looks generous in the shop gives them almost no usable route.
Bar spacing should be about half an inch, close enough that a small, slim head cannot push between bars, and horizontal bars on at least some sides help these light climbers move around the cage. A grate or easily cleaned solid floor matters more than in many cages, because Bourke's parakeets spend real time on the ground picking through food, and a floor that stays damp or dirty works directly against a ground-feeding bird's health.
Furnishing a Bourke's cage for crepuscular flight
Keep the central flight path open. Place the main perches at each short end of the cage so the birds fly the full length between them, rather than filling the middle with hanging toys that shorten every flight to a hop. Use natural branches of varied thickness, add a low perch and a flat resting spot near the floor for ground time, and put food and water at one end so droppings from the favourite roost do not fall into them.
Bourke's parakeets are busiest at dawn and dusk, so cage placement should honour that clock. A calm room that gets gentle morning light but is not blasted by midday sun suits them, and the household should expect soft twittering flights while people are having breakfast and again as the light fades, with long, quiet rests between. Covering the cage for a consistent, dark night keeps that rhythm steady.
One bird, a pair, or a shared aviary
A single Bourke's parakeet in the minimum cage still needs daily out-of-cage flight in a bird-safe room, because cage length alone cannot replace real flying for a bird kept without a companion's company on the wing. A pair uses a longer cage far better and encourages the natural paired flights that make this gentle species so watchable. Bourke's are famously peaceful and often share spacious aviaries with finches and other grass parakeets, but shared housing raises the floor-space requirement, not just the perch count, since every bird in a mixed aviary also forages low.
Avoid round cages, which offer no corner to retreat to and no straight flight line, and avoid cages marketed by height for climbing parrots. If you are choosing between two cages at the same price, the wider, plainer flight cage beats the taller, decorated one for this species every time.
A quick fit check before you buy
Measure the space at home first, including stand height and the door swing you will use for cleaning, then check the cage itself: sliding tray, secure feeder doors, and bar spacing you have measured rather than trusted from the label. Note the final dimensions, bar spacing, and where perches and dishes sit in the bird's own record, because with KinsLe's Transferrable Profiles a Bourke's parakeet changing keepers can arrive with its proven cage size and layout already listed, so its next home is set up right before it walks in.