Academic journal article The American Midland Naturalist

Temporal Patterns in Use of an Iowa Woodlot during the Autumn Bird Migration

Academic journal article The American Midland Naturalist

Temporal Patterns in Use of an Iowa Woodlot during the Autumn Bird Migration

Article excerpt

ABSTRACT.-

Migration is an important part of many temperate bird species' annual life history. We used a time-point census method to describe the changes in bird communities during the autumn migration in central Iowa. Site selection within a second-growth forest varied significantly based on diet and migration habits of birds. Temporal changes in individual activity and species presence show preferences that may be related to availability of fruit. We discuss the effect of the invasive shrub Rosa multiflora on bird abundances on both temporal and spatial scales.

INTRODUCTION

Nearly 300 species of North American birds engage in some form of seasonal migration, which is considered to be driven by diminishing food availability as winter takes hold in the northern latitudes (Cox, 1985). This mass movement of birds places increasing pressure on the resources within the "stopover" habitats that are visited by migrating birds (Winker et al., 1992), although the requirements of birds using these habitats-as well as the relative importance of specific types of habitat-are not well known (Weisbrod et al., 1993). For example, they may be seeking specific types of food with a particularly high lipid content in an effort to acquire extra body fat to accommodate the energetic demands of long distance, non-stop flight (Snow, 1970; Slithers et αι., 2000). Such dietary plasticity has been shown by Parrish (1997) in a study of migrants on Block Island, 19 km off of the coast of Rhode Island, USA; recaptured birds showed increases in body mass, indicating that the site was used for body fat accumulation. Birds can satiate their demand for high-lipid food through the consumption of fruit, leading to important interactions between fruit-producing plants and birds as agents for their dispersal (Snow, 1970; Thompson and Willson, 1979).

An intensification of frugivorous activity during the autumn migration may have important implications for habitat use and responses of birds to habitat change. For example, Suthers et al. (2000) showed that both migrant and resident birds, with the exception of the thrushes (Turdidae), preferred shrub lands and avoided areas that have become significantly dominated by a forest canopy in New Jersey, USA. Other studies indicate that birds migrating in the autumn may select their stopover habitats based on the availability of food capable of meeting their high-energy requirements. Additionally, those habitats which satisfy the demands of the migrating birds have been shown to experience a peak in avian activity late in the growing season. Weisbrod et al. (1993) used mist net studies along a stretch of the St. Croix River in Wisconsin and reported a peak of migrant birds in late September. Stapanian (1982) described a sharp peak in the mean number of frugivores spanning late September through late October using weekly censuses at his 16 census stations in Kansas, USA, although the distribution of birds at each site was highly variable. Winker et al. (1992) used mist nets along the St. Croix River in Minnesota to demonstrate that autumn migrants frequent woodland patches and put an acute seasonal demand on the resources within stopover habitats. Echoing Stapanian (1982), this study also suggests that habitat diversity is important in determining the role of stopover forest fragments.

Here we report on changes in avian communities in a second-growth woodlot in central Iowa between July and October 2003. We sought to extend the understanding of temporal changes in the autumn bird migration and its effect on local stopover habitats as called for by Weisbrod et al. (1992) and Winker et al. (1992). We describe the temporal changes in the avian community of a second-growth forest in Iowa, portions of which were being grazed up until the 1960s and row-cropped as recently as the 1980s. Using a time-point census design, we recorded all birds seen and heard from all levels within the habitat. …

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