The noisy scrub-bird is endangered and confined to one natural population on Mount Gardner and several translocated populations east of Albany, Western Australia. Annual monitoring of the number of singing males at Mount Gardner since 1966 indicates a steady rise in numbers, peaking at 179 birds in 1994 followed by a less regular decline to 19 by 2017. The Lakes, an adjacent location, had a similar pattern of increase from the early 1970s then irregular decline from the mid-1980s. Broken stick regression models confirmed that both apparent patterns represented significant increase then decline in the number of singing males. Territory size estimates from 1994, 2005 and 2014 suggest an increase in size of territories over time but that pattern was not significant when only common territories were compared. Variance in bird numbers was not correlated with annual rainfall, which has broadly declined over the study period, or with number of rain days. We discuss five possible causes of decline: (1) inbreeding, (2) control of exotic predators and associated impact on non-target, natural predators, (3) resource/food availability (which may interact with rainfall), (4) removal of birds for introduction programs, and (5) the absence of wildfire and associated changes (senescence) in vegetation. None are compelling, single explanations for decline but their collective impact may be high. Survival of the noisy scrub-bird may be increasingly dependent on the introduced, managed populations of this species at Mount Manypeaks and on Bald Island.