Illinois Natural History Survey - University of Illinois

Eastern Bluebirds: Effects of Environment on Reproductive Strategies

Anyone who studies bluebirds hears the same comments: "I haven't seen a bluebird in thirty years!", or "Are bluebirds still around?" In the early 1900s, bluebirds were a common sight in suburbs and rural areas. By the middle of the century, however, populations had shrunk to one-tenth of their original numbers. Several factors contributed to this decline, including loss of nest sites due to new agricultural practices (trimming dead snags and replacing wood fence posts with metal) and competition with introduced starlings and house sparrows. Only with the construction of bluebird trails, consisting of nest boxes erected and monitored for bluebird use, have populations rebounded in recent years.

The history of the bluebird is a good example of how the physical habitat and biotic community in which animals live affect their survival and fitness. For birds, habitat structure determines the availability of nest sites, availability and accessibility of food resources, and the types and densities of predators and nest-site competitors. In addition, because environmental conditions vary among habitats over time, and may alter the success of a given reproductive strategy, there must be some degree of behavioral flexibility involved. All of these factors influence the decisions that breeding birds make, as well as their ultimate fitness. By examining links between reproductive performance and the breeding environment, this de-cision-making process and the development of life histories can be better understood.


Male eastern bluebird (Sialia sialis).

A study is currently under way to determine how environmental factors influence the reproductive behavior and success of the eastern bluebird, a cavity-nesting songbird, in central Illinois. From 1992 to 1994, reproductive traits, behavior, and success of box-nesting bluebirds in three different habitats (early successional, agricultural, and forest edge) were monitored. We found that nesting habitat did not seem to affect reproductive strategies and there was no difference in the proportion of boxes used in each habitat group. In addition, neither clutch size nor nestling mass (an indicator of nestling quality) differed across habitat groups. The groups, however, did differ in terms of productivity. Early in the season, nests along forest edges produced a lower proportion of fledglings than those in the other groups. This is probably due to nest-site competition from house wrens that set up territories during this time, and because shrubby and forest edge habitats are a known source of nest failure in bluebirds. Late in the season, nests in agricultural areas were less productive than those in other habitats. During this time, wren aggression decreases as they raise their own young. Also, vegetation in agricultural habitats is tall and dense, limiting bluebird foraging ability and increasing cover for nest predators.

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Researchers capturing an incubating female bluebird to be weighed and banded.

To further document the effects of environmental factors on avian reproductive strategies, an experiment is currently under way that manipulates food availability and risk of nest predation. Active nests are assigned to one of four treatment groups: supplemental food/increased nest predation risk, supplemental food/normal nest predation risk, normal food availability/increased nest predation risk, and a double control. At nests receiving supplemental food, 200 mealworms are provided near the nest every other day during the nestling. Nest predation risk is simulated by presenting a nest predator model (rubber snake) on and near the box several times during the nestling phase. In addition to monitoring reproductive success, behavioral watches are done to assess the effect of the treatments on parental foraging and nest- guarding behaviors. Comparing reproductive success and parental behaviors among treatment groups will help us to understand the strategies used by parents under different environmental conditions. This study began in 1995 and will conclude in 1996.

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Brood of five bluebird eggs in a Peterson-style nest box.

As more and more land is modified and disrupted by human use, it is beneficial to know how these changes will affect populations of plants and animals. Bluebird productivity is affected by the surrounding habitat, both natural and man-made. Cavity-nesters are often nest-site limited, and thus may be forced to breed wherever there are adequate nest sites. Other species may be affected in different ways, perhaps in their behavior or long-term fitness.

Laura R. Lee, Department of Ecology, Ethology, and Evolution, University of Illinois; Jeffrey Brawn, Center for Wildlife Ecology



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Last Modified 7/03/96



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