Geology controls the distribution of a seed-eating bird: Feeding-tree selection by the glossy black-cockatoo Calyptorhynchus lathami

Abstract

Despite seed production being nutrient-limited, the influence of nutrient pathways on granivore distributions is unclear. This article examines the influence of geology and soil on the distribution of glossy black-cockatoos (Calyptorhynchus lathami), which feed almost exclusively on the kernels of casuarinas (Allocasuarina spp. and Casuarina spp.), and are selective about the trees in which they feed. To clarify the basis of this selection, Food Value (a measure of dry matter intake rate) and kernel nutrient content were compared between feeding and non-feeding trees of drooping sheoak (A. verticillata). Random forest modelling was then used to examine the influence of geology and soil chemistry on Food Value. Finally, logistic generalised additive modelling was used to examine the influence of geology on cockatoo feeding records across the range of black sheoak (A. littoralis) and forest oak (A. torulosa), drawing on a statewide dataset. Food Value–but not kernel nutrient concentrations–influenced feeding tree selection. Soils under drooping sheoak were nutritionally poor, with low nitrogen and phosphorus (despite high concentrations of these nutrients in the kernels), and characterised by two principal components: SALINITY (dominated by exchangeable magnesium and sodium, electrical conductivity, and sulphur) and ACIDITY (pH, iron, and aluminium). Random forest modelling showed that Food Value was highest on sedimentary rocks, with a high ACIDITY score, less than 18 meq 100 g-1 exchangeable calcium, and less than 4% soil organic carbon. The odds of cockatoos selecting casuarinas as feedings tree were three times higher on non-calcareous sedimentary rocks than on other rock types. Non-calcareous sedimentary rocks produce low-fertility, acid soils, which promote nitrogen-fixation by Frankia. I therefore conclude that glossy black-cockatoo distribution is controlled by the casuarina’s symbiotic relationship with Frankia, which is ultimately controlled by geology; and that similar relationships may be responsible for the prevalence of several other species on low-fertility and/or acid soils.