Use of remnant forest habitat by birds during winter in subtropical Australia: patterns and processes

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The south-east Queensland region of subtropical Australia supports a high diversity of landbird species, many of which are migrants which visit lowland areas during winter. However, lowland habitats have been impacted by clearing and fragmentation of native forest types within the past 170 years. This paper considers the implications of loss and fragmentation of lowland eucalypt forests for birds in winter, by comparing the proportionate density of twelve selected species across 49 different cases representing a variety of structural habitat differences. There was little fragmentation effect on forest-dependant winter immigrant species down to about 10 ha. The smallest remnants had reduced proportionate abundances of forest-dependent species together with elevated abundances of two other species: Noisy Miner and Australian Magpie. Similar shifts in species composition occurred in association with a variety of types of change in native eucalypt forest, all involving alteration to physical structure incorporating reduction of foliage cover (at forest edges, in areas without understorey, in regenerating forest, on slopes and ridges, and after fires). We consider the ecological processes that might underlie these patterns, and suggest that: (1) habitat selection, possibly mediated and/or reinforced by interference competition from Noisy Miners, is a key process in the study system in winter; (2) habitat loss, rather than fragmentation, is likely to be the main cause of regional declines in forest-dependent winter migrants; and (3) the management of habitat for winter migrant birds within lowland eucalypt forests of this region should focus on maintaining and improving: (a) the overall percentage of lowland area covered by native forest, and (b) factors associated with the structural integrity (canopy and understorey cover, fire frequency, retention of complete microtopographic gradients) of remnant forest areas irrespective of their size, at least down to about 10 ha.