News Release

Fragmented Forest, Fragmented Food

Peer-Reviewed Publication

Ecological Society of America



Female Yellow Robin on nest. Photo taken by Liana Zanette. May be re-used without permission, but should be credited as follows: "Liana Zanette, University of British Columbia"

Full size image available through contact

Each year, as the world's forested areas become more and more fragmented, ornithologists become more and more alarmed. Many forest songbirds occur less often than expected in small forest fragments. Part of the problem, researchers determined a few years ago, is that forest fragments and edges contain larger numbers of brood parasites, such as the brown-headed cowbird. Now, a team of scientists working in Australia has also discovered that forest fragments may not offer enough food for some songbirds.

Working in forested areas in the southeastern corner of Australia, Liana Zanette (University of British Columbia) and her two colleagues, Paul Doyle and Steve Tremont (University of New England in New South Wales) studied the nesting and feeding habits of the Eastern Yellow Robin (Eopsaltria australis) in forest fragments both large and small. As a part of their study they also examined the type and amount of food available to these birds in the different sizes of forest plots, to see if food would make any difference to the birds' ability to successfully reproduce. The results of their study appear in this month's Ecology (volume 81 issue 6) , a journal published by the Ecological Society of America.

Yellow Robins have a reproductive life history similar to many of the forest-interior migrant birds currently of conservation concern in North America. They lay relatively small clutches of eggs and have short nesting cycles. Their nests are often found in tree forks or bushes about three meters off the ground. While female Yellow Robins incubate their eggs, male Yellow Robins forage for food. The male will bring food to his mate and will also bring food to the young nestlings once they have hatched. Occasionally, the female will leave the nest to search for her own food. She usually also helps find food for the nestlings late in the nesting period. Yellow Robins employ a "sit and wait" foraging tactic to capture insects and small lizards on the ground by pouncing on their prey from above.

Zanette and her team used small pitfall traps to measure the amount of prey potentially available to these robins. Using PVC piping and alcohol, the researchers trapped insects and small skink lizards in forest fragments of differing sizes.

The team also spent time observing the birds and recording their behavior during several key points in the nesting cycle. Birds were also caught in mist nets, banded for individual recognition, and then released into the forest.

The team found that although the same variety of food was available in the small forest fragments, the amount of potential prey was significantly lower. In some areas, there was two times more food in the large fragments than in the smaller.

Because of this food shortage in the smaller fragments, females incubating their nests received 40 percent fewer feeding visits from their mates. In addition, females were two times more likely to make at least one foraging trip away from their nest than their counterparts in the larger fragments. Females in the larger fragments also bred for an average of three weeks longer than their counterparts in smaller fragments.

Although the number of food items brought to each nestling did not vary with fragment size, the size of the individual food items differed greatly. Nestlings in the small fragments were three times less likely to receive large prey than those in the large fragments. Consequently, nestlings received less food volume in the small fragments than did those in the large fragments.

The number of eggs in each clutch did not differ between the contrasting areas of forest which were studied, but the eggs in the larger fragments were on average 7 percent heavier than those in the small fragments. Nestlings in the smaller fragments also tended to have smaller body mass.

In future studies, Zanette would like to investigate the role that this reduced food availability may play in increasing vulnerability to nest predation.

"Adults spend less time guarding their nests when food is in short supply so predators may have more chances to rob the nest," she hypothesizes. "Nestlings that are not well fed also beg for food more vigorously, which may also increase the chance of attracting attention from predators."

Researchers are still not certain why small fragments have fewer invertebrates than large fragments, but Zanette believes that the small fragments have different microclimates. Edges of the forest tend to have drier soil and leaf litter and insects prone to desiccation may be excluded from these areas, she says.

"In addition to understanding whether food supply varies with fragment size, we need to know why it may vary, " says Zanette. "A clear connection between microclimate and invertebrate biomass needs to be made."

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*Photo available: please visit ESA's website at: http://esa.sdsc.edu/press.htm or contact Alison Gillespie.

Ecology is a peer-reviewed journal published eight times a year by the Ecological Society of America (ESA). Copies of the above article are available free of charge to the press through the Society's Public Affairs Office. Members of the press may also obtain copies of ESA's entire family of publications, which includes Ecology, Ecological Applications, and Ecological Monographs. Others interested in copies of articles should contact the Reprint Department at the address in the masthead.

Founded in 1915, the Ecological Society of America (ESA) is a scientific, non-profit, organization with over 7000 members. Through ESA reports, journals, membership research, and expert testimony to Congress, ESA seeks to promote the responsible application of ecological data and principles to the solution of environmental problems. For more information about the Society and its activities, access ESA's web site at: http://esa.sdsc.edu .


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