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By Margot Higgins

Food is scarce for the eastern yellow robin,
Eopsaltria australis, when forests are fragmented.

Fragmented forests may not provide enough food for certain songbird populations, according to a recent study.

Working in fragmented forest areas in southeastern Australia, zoologist Liana Zanette and colleagues Paul Doyle and Steve Tremont studied the nesting and feeding habits of the eastern yellow robin. Their findings are published in the June issue of the journal Ecology.

Since the early 1980s, scientists have been keenly interested in the severe decline of songbird species throughout the world. Several studies have shown that high nest predation and brood parasitism in fragmented landscapes are to blame for the loss of songbirds. Yet few reports factored food into the equation.

Eastern yellow robins, an endemic species to Australia, have similar reproductive patterns to many other birds in peril, according to Zanette. "They have all of the characteristics of other species that aren't going to hang around," she said.

The researchers began their work by examining the type and amount of food available to robins in forest fragments of different sizes. Using small pitfall traps, they measured the amount of prey potentially available to the robins.

Although the same variety of food was available in smaller forest fragments, the amount of potential prey was significantly lower. Many of the larger forest fragments offered twice as much food as the smaller fragments, Zanette said.

Liana Zanette is a postdoctoral
fellow at the University of British
Columbia in Vancouver, Canada.

Next, the researchers investigated how a reduced diet might impact the birds' ability to reproduce successfully.

Female robins in small forest fragments received 40 percent fewer feeding visits from their mates. In larger fragments, female robins bred three weeks longer on average than those living in smaller fragments.

Nestlings in small fragments were cheated on dinner: They were three times less likely to receive large prey than those in large areas. Consequently, they received less food overall.

While the number of eggs in each clutch did not differ, the researchers found that eggs in larger fragments were 7 percent heavier than those in small fragments. Nestlings in the smaller fragments also tended to have smaller body mass.

Zanette said her research might push wildlife managers to consider adding food sources to certain bird population areas. In future studies, she plans to investigate the role that reduced food sources may play in vulnerability to nest predation.

"We found that females left the nest more frequently in the smaller fragments," Zanette explained. "If the female is off the nest, there is no one around to guard it from potential predators. Nestlings that are not well-fed also beg for food more vigorously, which may also increase the chance of attracting attention from predators."