Avian Habitat Evaluation: Should Counting Birds Count?

There are times when birds reproduce at higher rates in places where they are less abundant, limiting the generally accepted value of bird counts as environmental indicators. But how often, and under what circumstances, does this happen? In 109 published cases involving 67 species across North America and Europe, higher density sites displayed greater recruitment per capita and per unit of land area in 72% and 85% of cases, respectively. The frequency of negative relationships between abundance and reproductive success did not differ between different kinds of birds or habitats. However, density was negatively related to reproductive success more often in areas of human disturbance than in relatively natural areas. Although further study is needed to confirm the generality of this pattern, especially in areas such as the tropics, results suggest that birds can fail to recognize ecological traps or opportunities in landscapes that differ from those in which they evolved.