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Whitfield (2000)

Hooded Warbler Wilsonia citrina

Pennsylvania

Stutchbury (1997)

aCowbird removal became part of the conservation management undertaken for these species (see also De Groot et al. 1999, Rothstein & Cook 2000).

aCowbird removal became part of the conservation management undertaken for these species (see also De Groot et al. 1999, Rothstein & Cook 2000).

no longer to be producing enough young to offset known adult mortality. In a mainly agricultural landscape in central Illinois during 1985-1989, nests of forest-nesting Neotropical migrants contained an average of 3.3 cowbird eggs per nest. End-of-season juvenile to adult ratios averaged 0.1 in Neotropical migrants compared with more than 1.0 for year-round residents and short-distance migrants (Robinson 1992). The Neotropical migrants could not have maintained their numbers under this level of parasitism without large-scale immigration.

It thus seems, then, that many North American songbirds that nest in forest fragments in suburban and agricultural areas are now experiencing the dual effects of increased nest predation and parasitism, particularly near forest edges, and that for various reasons these affects fall most strongly on many Neotropical migrants. So far, attempts to assess these impacts on population levels have been made only in a minority of species, such as the Kirtland's Warbler Dendroica kirtlandii and Wood Thrush Catharus mustelinus, in which survival and breeding success have been measured. Studies on a greater range of species, and in a greater range of areas, are needed before the generality of these findings can be assessed. Although the predisposing fragmentation of the eastern deciduous forest occurred mainly in the nineteenth century, the associated increase in generalist predators and cowbirds occurred mainly in the latter half of the twentieth century. Decline through excess predation and parasitism would be expected to leave increasing amounts of breeding habitat unoccupied.

Forest fragmentation may also reduce the food available per unit area. This aspect has received much less attention than predation and parasitism, but could be important in some species. It was found among Ovenbirds Seiurus aurocapillus nesting in different-sized forest fragments in Ontario (Burke & Nol 1998). Density and pairing success of territorial males increased with area of the woodlot. Within Ovenbird territories, prey biomass was 10-26 times greater in large woods than in small ones, associated with deeper leaf litter. Similar results were also found for Australian Yellow Robins Eopsaltria australis whose food supply of ground-dwelling invertebrates was also lower in smaller woods (Zanette et al. 2000).

The relationship between forest area and species numbers was found to vary regionally: deficiencies in species numbers in small forests tended to be more marked in agricultural than in more forested landscapes (Freemark & Collins 1992). These regional differences were in turn associated with similar regional variations in songbird nest success. In nine migrant species, predation and parasitism rates were found to increase with reduction in the proportion of forest present. Landscapes with little forest acted as sinks for some species, in that reproduction was insufficient to offset annual mortality, so that breeding numbers could not be maintained in the absence of continual net immigration from more forested areas (notably Wood Thrush Catharus mustelinus, Hooded Warbler Wilsonia citrina, Ovenbird Seiurus aurocapillus and Kentucky Warbler Oporornis formosus). There were some exceptions to the trends, as local factors - including those influencing cowbird abundance - modified the overall patterns (compare the studies on Wood Thrushes in Illinois, Indiana, Pennsylvania and Delaware, Hoover et al. 1991, Robinson 1992, Roth & Johnson 1993, Weinburg & Roth 1998, Fauth 2000).

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