Nesting Dynamics of the Eastern Bluebird (Sialia sialis) in Southeastern Illinois

Data collected from 1981 through 1988 from an Eastern Bluebird (Sialia sialis) nesting box trail, involving 512 nests and 2176 eggs, in Crawford County, Illinois, were analyzed and compared to similar published works. The average nesting season lasted 167.4 days, ran from 17 March through 31 August, and was divided into three breeding peaks (or phases). Most breeding pairs were observed with either one or two clutches per season; however, triple-brooded pairs were also present, the clutches of which coincided well with the three peaks. Most data showed a decrease as the season progressed. For example, mean clutch size was 4.4 eggs (4.8 in phase 1, 4.3 in phase 2, and 3.8 in phase 3); incubation lasted about 14.5 days (15.0 days in phase 1, 14.4 days in phase 2, and 14.3 days in phase 3); the mean number of eggs hatched was 3.2 (3.5 in phase 1, 3.2 in phase 2, and 2.9 in phase 3); the mean number fledged was 2.8 (3.0 in phase 1, 2.8 in phase 2, and 2.4 in phase 3); and about 17 days passed between hatching and fledging (17.4 in phase 1, 17.0 in phase 2, and 17.1 in phase 3). Over 79% of all clutches hatched at least one egg, while 70.8% of all clutches fledged at least one young. Nestling sex ratio was 1:1. Although bluebird eggs are usually blue, 3% of those laid in this study were white. Of all eggs laid, 33.8% failed due to various reasons, of which raccoon predation (28.6%), house sparrow competition (16.3%), and infertility (14.1%) were the major causes. Plastic bleach jugs were more successful nesting sites than the conventional wooden box, especially when dealing with sparrow competition. Eight percent of all birds banded returned to the area in a following year. More birds banded as adults (21.2%) than those banded as nestlings (5.6%) returned, but no difference was found between the sexes of returning birds. Overall, the bluebird population analyzed in this study was typical of the species