Conservation of Declining Populations

The conservation of declining populations focuses on the ecological causes of the population decline. This approach preserves populations that are still large, but declining, by studying the species history and determining the reasons for decline and species needs for survival.

The rebound of the Eastern Bluebird in the United States is one successful example of this approach. Bluebirds are secondary cavity nesters, meaning they rely on natural cavities, abandoned nesting sites of other species, or artificial cavities, such as old fence posts. 

Eastern Bluebird populations began to dwindle in the 1950s, reaching minimum levels in the 1960s. This decline is attributed to several factors, including pesticides and invasive, non-native species. 

House sparrows, introduced in the 1950s, frequently outcompete bluebirds for nesting cavities, often with deadly consequences. These threats compounded natural pressures, such as harsh winters, predators, and competition with native species.

In the 1960s, designated trails were outfitted with nesting boxes to provide cavities for bluebirds. Scientists and volunteers monitored the boxes, removing invasive competitors and evaluating the breeding success of different nesting box configurations.  

This work identified optimal features for bluebird nesting boxes that reduce the competitive advantages of non-native, invasive species. For example, an entry hole with a maximum width of 3.8 centimeters prevents disturbance by European Starlings. 

These conservation efforts have helped Eastern Bluebird populations recover and actually increase in many areas. This success story demonstrates that, while human activities often contribute to species decline and endangerment, conservation efforts can help these populations recuperate.