Is more better: Counting birds may only tell part of the story

October 07, 2004

Bird counts can offer an initial assessment of bird populations, but don't necessarily provide the full picture. In "Avian habitat evaluations: should counting birds count?" Carl Bock and Zach Jones (University of Colorado -Boulder) reviewed studies from the past 20 years, looking at whether a large number of birds in an area meant the birds would successfully reproduce and survive.

Their study was inspired by an article written by Beatrice Van Horn in 1983, where she initially suggested that density could be a misleading indicator of habitat quality if it was disconnected from reproductive success or survival.

After reviewing multiple studies, Bock and Jones found that counts are generally good indicators, but that there are some disconnects between abundance and reproductive success. They also discovered a knowledge gap in understanding the relationship between abundance and survival in birds.

This information has been vital in management decisions involving animal conservation. A large population of birds in a region could indicate a suitable habitat, or it could, in fact, could be the only place the animals could find, wrong for feeding, mating, and rearing young, in other words, an "ecological trap."

"Ornithologists and ecologists in Europe and North America have good reason for using bird count results as indicators of environmental conditions - at least in terms of breeding habitat quality," say the researchers in their report. Variation of habitats and types of birds could influence the relation between abundance and reproductive success. Out of the studies examined, disturbed areas are more likely to show negative associations between abundance and reproductive success.

The authors provide several examples of the impact of human disturbances on bird populations.

The black-throated sparrow in southern New Mexico prefers mesquite savannas, but has low reproductive success in these areas, probably because of livestock grazing on the grassy undergrowth. Northeastern US songbird populations have been shown to fare better in forest interiors than forest edges or fragmented forests. Bock and Jones suggest that, over time, some species adjust to these disturbances. In southern France, Eurasian eagle owls were more productive and abundant in areas where long-term human activity maintained an open habitat and abundant prey. This suggests some birds are able to adjust to changes, seizing the "ecological opportunities" that may arise from human disturbances and learning to avoid ecological traps.

For a global understanding of these results, Bock and Jones suggest that studies outside Europe and North America, in areas such as the tropics, are needed to provide more insight into the patterns observed in this study.

Overall, "birds are usually able to aggregate in the higher quality breeding locations, regardless of the type of bird or habitat," say the authors.

"Avian habitat evaluations: should counting birds count?" appears in the October issue of Frontiers in Ecology and the Environment.
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Ecological Society of America

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