Songbird Nesting Ecology in the Pine Barrens of Northwestern Wisconsin

Abstract

Grassland and shrubland bird species of the East and the Midwest have declined at greater rates than any other avian guild in North America. In northwestern Wisconsin pine barrens, prescribed fire has been used to maintain early successional savanna habitat previously created by wildfire. However large-block timber harvest is being used more frequently to provide similar early successional habitat. I evaluated songbird nesting ecology in wildlife areas managed with prescribed fire and areas recently harvested due to a jack pine budworm (Choristoneura pinus) outbreak during the 1990s. I examined site selection and success of nests in both management types. In 2000 and 2001, 176 nests were located on reserves managed with fire and 182 were located on clearcuts and followed until nest fate was determined. Vegetation measurements were taken at all nests to determine factors influencing nest-site selection and nest fate. Discriminant function analyses were used to determine nest site characteristics of 8 species based on microhabitat and fine-scale vegetation measurements. Clay-colored sparrow nest placement in managed areas was more variable than in unmanaged landscapes, where nests were almost exclusively located in shrubs. Yellow warbler nests were typically found in very dense stands of hazelnut with high visual obstruction measurements and a high amount of live vegetation. Brown thrashers and Eastern towhees selected areas on the ground usually centered under a willow or similar shrub species. Other ground nesting species, including Brewer's blackbirds, song sparrows and vesper sparrows, selected areas with more grass, and less green vegetation. Classification success was lower for these species, indicating that they nest in similar areas. The apparent influence of disturbance type on nesting success changed depending on the analysis. Raw nesting success (successful nests/ nests found) indicated that disturbance type did not influence reproductive success. When exposure days (the amount of time nests were observed) were examined, nesting success differed among species and between disturbance types. However, the effect of disturbance type was reduced or removed when vegetation and landscape characteristics were also considered. Clay-colored sparrows and Brewer's blackbirds had higher nesting success in unmanaged areas, whereas yellow warblers and Eastern kingbirds had higher nesting success in managed areas. Song sparrows and Eastern towhees also had higher nesting success on managed areas, but results are based on limited sample sizes of 19 and 26 nests, respectively. All nests were examined for presence of brown-headed cowbird eggs at each nest visit. Parasitism rates by brown-headed cowbirds were very low on both managed and unmanaged sites (0.032, and 0.01 respectively). Vegetation and landscape characteristics were used to construct logistic exposure models attempting to determine factors influencing songbird nesting success. The amount of time a nest was observed was built into the logistic regression models. Distance from the nearest tree or distance from the forest edge influenced nesting success in 6 of the 8 species examined. Although I did find differences in Mayfield nesting success between management types for some species, management type was not a strong factor influencing nesting success in logistic exposure models that incorporated landscape and vegetation characteristics. Incorporating large scale timber cuts in the pine barrens will likely provide early successional habitat many of these bird species can utilize and successfully produce offspring.