A hypothesis to explain why the south-western subspecies of the Crested Shrike-tit (Falcunculus frontatus leucogaster) is rare and declining

Abstract

The south-western subspecies of the Crested Shrike-tit (Falcunculus frontatus leucogaster) is uncommon and declining in abundance. Moreover, populations fail to persist in even large remnants of native vegetation within agricultural areas. Reasons for the scarcity of Shrike-tits in south-western Australia, their decline, and their failure to survive in fragmented habitats are unclear, but are possibly related to the scarcity of persistent decorticating bark on south-western Australian eucalypts relative to eucalypts in habitats where the species is abundant in eastern Australia. At two widely separated areas of eucalypt forest and woodland in eastern Australia, where Shrike-tits have been studied and appear relatively abundant, they take most of their prey from bark, particularly loose and decorticating bark. By contrast, in woodlands of south-western Australia, most prey appears to be obtained by foliage gleaning. I suggest that bark substrates provide a richer food resource for Shrike-tits than foliage, and that Shrike-tits in south-western Australia must forage over much larger areas to obtain adequate food than those that have been studied in the east. Other studies have shown that canopy arthropods are less abundant on eucalypts in south-western than in eastern Australia. Thus, as a result of both differences in foraging substrate availability and lower arthropod abundances, Shrike-tit population densities are lower in south-western Australia than much of eastern Australia. Low population densities and the need for large home-ranges typify organisms sensitive to habitat fragmentation. It is therefore hypothesised that both low numbers and a failure to persist in habitat fragments by Shrike-tits in southwestern Australia are a consequence of the limited abundance of suitable bark substrates for foraging, a reliance on foliage gleaning, and lower arthropod abundances. Conservation of this species in south-western Australia therefore requires protection of large contiguous areas of eucalypt woodlands and forests.