Carnaby’s cockatoo, Calyptorhynchus latirostris, is endemic to south-western Australia. It has undergone a major decline in range and abundance as a result of extensive removal of breeding and foraging habitat. It is now classified as endangered by the State of Western Australia, the Australian government, and internationally. In order to plan effective conservation management for the species it is important to assess the success of breeding populations throughout its range. In this paper we examine the efficacy of using the relationship between growth in the length of a nestling’s folded left wing and body mass to assess nestling condition, and examine known breeding failure in relation to nestling condition in two breeding populations: one at Coomallo Creek (studied from1970–2017), and the other at Manmanning (1969–76).Results demonstrated that the lighter the nestlings, the higher the rate of breeding failure. Data from the Coomallo Creek population were used to prepare a table of nestling folded left wing length and body mass as a benchmark for assessing nestling condition in 10 other breeding populations, based on data collected from 1970 to the present. Following extensive clearing that removed foraging and breeding habitat, two of the 10 populations produced nestlings that were significantly lighter than the benchmark, and both populations subsequently declined to extinction. The commencement of egg-laying each season at Coomallo Creek was strongly influenced by total rainfall in the first half of autumn. The length of the egg laying period between 1970–76 and 2009–17 increased by 5.2 weeks (40%). This increase was related to changes in rainfall and temperature over more than four decades. Despite the lengthening of the egg-laying period, nestling condition was unaffected, suggesting that, at least in the short term, the Carnaby’s cockatoo population at Coomallo Creek is coping with the effects of climate change.