Woody plant encroachment drives the decline of a grassland bird: The fate of golden-shouldered parrot (Psephotellus chrysopterygius) nests.

Grasslands are the world's most threatened terrestrial biome, with consequences for grassland-dependent species. Many remnant grasslands are threatened by woody plant encroachment (hereafter "encroachment"). Several studies have found that encroachment eliminates grassland species through increased predation rates at the nest. The golden-shouldered parrot Psephotellus chrysopterygius is an Endangered species found on Cape York Peninsula, Australia. Even though this species mainly nests along grassy drainage depressions, a previous study concluded that encroachment improves its nest success. We tested this proposition by assessing the fate of 555 eggs laid in 108 nests. We tested the impact of stem density on the fate of eggs, chicks and adults, and on predation events using linear-by-linear association tests; and on nest success using logistic exposure regression. We then compared the contraction of the parrot's nesting distribution over three decades with change in canopy foliage cover. We also examined whether nest location was influenced by stem density, and explored the processes driving encroachment across the parrot's distribution, particularly fire frequency, which has been shown to influence encroachment on Cape York Peninsula. The parrots preferentially nested in areas of low woody vegetation density. In contrast to previous work, we found that encroachment increased the probability of predation, and reduced nest success and survival of nesting adults. Encroachment both drove a decline in fire frequency and was exacerbated by it. The parrots have abandoned areas where encroachment has been most advanced. This study provides an Australian example of the negative effects of encroachment on nesting success that have been demonstrated in many species from North American prairies and other grassland habitats. It supports the current management efforts to reverse encroachment in the parrot's habitat. We conclude that, wherever woody plant encroachment is occurring, it should be considered as a potential threatening process and managed accordingly.