Winter conditions influence biological responses of migrating hummingbirds

Conserving biological diversity given ongoing environmental changes requires the knowledg of how organisms respond biologically to these changes; however, we rarely have this information. Thi data deficiency can be addressed with coordinated monitoring programs that provide field data acros temporal and spatial scales and with process-base models, which provide a method for predicting ho species, in particular migrating species that face different conditions across their range, will respond t climate change. We evaluate whether environmental conditions in the wintering grounds of broad-Taile hummingbirds influence physiological and behavioral attributes of their migration. To quantify winte ground conditions, we used operative temperature as a proxy for physiological constraint, and precipitatio and the normalized difference vegetation index (NDVI) as surrogates of resource availability. W measured four biological response variables: molt stage, timing of arrival at stopover sites, body mass, an fat. Consistent with our predictions, we found that birds migrating north were in earlier stages of molt an arrived at stopover sites later when NDVI was low. These results indicate that wintering conditions impac the timing and condition of birds as they migrate north. In addition, our results suggest that biologicall informed environmental surrogates provide a valuable tool for predicting how climate variability acros years influences the animal populations.