Abstract
We have monitored breeding bird densities over a variety of sites and habitats in GTNP since the early 1990s, utilizing fixed-area census sites of around 5 ha in size. The sites are located throughout the park in all habitat types and over a wide range of elevations, and number 30 in all. At some of these monitoring sites we have accumulated data in successive breeding seasons for almost two decades; the power of these census data in interpreting variation in bird species composition and breeding densities, species to species, site to site, and especially year to year, clearly increases with the span of the data set. Some of the measured variation in breeding densities is presumably attributable to conditions encountered by resident birds during the preceding winter, on-site in GTNP. Some may be attributable to conditions evaluated by migrant birds returning to GTNP after wintering elsewhere, also an on-site contribution. However, a further potential source of variation is off-site, and may be ascribed to conditions endured by the migrants on their wintering grounds. It is the source and extent of such variation in the winter habitats of GTNP migrants that is the subject of the ensuing discussion.