POPULATION-CHANGES IN BOREAL FOREST BIRDS IN SASKATCHEWAN AND MANITOBA

We counted breeding birds at four plots in central Saskatchewan and fo ur in western Manitoba in 1990-1992 to examine changes in species comp osition and abundance since the plots were originally surveyed in 1972 -1973. In Saskatchewan, more species of Neotropical migrants decreased (16) than increased (9; P > 0.05 < 0.1). Combined densities of Neotro pical migrants declined (14-44%) on all of the Saskatchewan plots; Ten nessee Warblers (Vermivora peregrina), Red-eyed Vireos (Vireo olivaceu s), and Ovenbirds (Seiurus aurocapillus) declined on the most plots an d by the greatest magnitude and Black-throated Green Warblers (Dendroi ca virens) and Rose-breasted Grosbeaks (Pheucticus ludovicianus) also decreased. Six of seven Neotropical migrants showed the same direction of change as in a province-wide Breeding Bird Survey. Successional ch anges did not account for decreased densities of these five species, b ut they may partly explain increases in some other species. Fluctuatio ns in food supply (e.g., spruce budworm [Choristoneura fumiferana]) co uld not explain changes, because some species that should have respond ed numerically to budworm outbreaks that occurred in the 1990s had inc reased whereas others had decreased at the same site. The surrounding forest remained continuous over the 17-18 years, so changes in forest area cannot account for the declines. In Manitoba, more Neotropical mi grants increased (19) than decreased (11) according to combined densit ies from four plots. Combined densities of Neotropical migrants also i ncreased at three of the four plots (33-123%). Ten of 15 Neotropical m igrant species showed different 'trends' than a province-wide analysis of BBS data. All changes in Manitoba could be attributed to vegetatio n succession on the plots and forest fragmentation in surrounding land scapes. Trends in Saskatchewan may be representative of general declin es within continuously forested boreal landscapes, whereas those in Ma nitoba may reflect reduced opportunities for breeding in continuous fo rest as the landscape is increasingly fragmented by agriculture.