We used long-term data on the demography and habitat quality of Black-throated Blue Warblers (Dendroica caerulescens) that were collected during 1996 - 2006 across a climate gradient within the Hubbard Brook Experimental Forest, New Hampshire to assess the potential effects of climate change on habitat quality and demography. The climate gradient occurs across an approximately 600-m elevational range and is characterized by a 2 ^oC difference in mean annual temperature and 1.5 cm difference in precipitation.  We found that spring weather determines the elevation at which birds settle and when egg-laying begins.  However, warmer conditions across the climate gradient were associated with lower habitat quality.  At lower elevation, breeding pairs had less food (sampled by visual censuses of caterpillars on leaves and Malaise traps), less foraging substrate (density of deciduous leaves in the shrub layer) and encountered more nest predators (5-min surveys).  In consequence, annual fecundity and apparent survival (return rates) were significantly less at low than at higher elevations.  Reduced habitat quality will affect demography, population size and dynamics, via these effects on annual fecundity and return rates.  The range in climatic difference that we studied, 2 ^oC, is less than the 2.5 to 4.1 ^oC increases projected for the northeastern U. S. region in the coming century.  We therefore conclude that projected warming is likely to have a significant negative effect on the demography of migrant birds in northeastern North America in the near term due to reductions in habitat quality.