Persistence of the threatened Knysna warbler Bradypterus sylvaticus in an urban landscape: do gardens substitute for fire?

Food resources can limit populations of insectivorous birds. Previous studies have shown strong correlations between bird population densities and their invertebrate food, with declining populations being attributed to loss of prey. This might also be the case for the threatened Knysna warbler ( Bradypterus sylvaticus ), which in the last 20 years has declined precipitously on the Cape Peninsula, South Africa. The bird is abandoning natural, protected forests, retreating to narrow belts of suburban, riverine woodland with dense, tangled understoreys. The potential invertebrate food availability in natural forests and suburban woodland was assessed against what is known of the bird’s preferred prey. The invertebrate food resource spectrum and abundance were determined using three sampling techniques in both natural forest and suburban woodland. Despite greater overall abundance of invertebrates in suburban woodland, the warbler’s preferred prey items were equally abundant in both habitats. This suggests that reduced food supply does not explain abandonment of natural forests by the warbler. Rather, it is concluded that the availability of suitable nesting substrata, which are more abundant in suburban woodland, has driven the biotope shift. This is an unusual case of a localized and threatened bird species faring better under transformed conditions than in natural habitats.