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,
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Contact: Annie Drinkard
annie@esa.org
Ecological Society of America
7-Oct-2004

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