Climate change is affecting altitudinal migrants and hibernating species

Calendar date of the beginning of the growing season at high altitude in th e Colorado Rocky Mountains is variable but has not changed significantly ov er the past 25 years. This result differs from growing evidence from low al titudes that climate change is resulting in a longer growing season, earlie r migrations, and earlier reproduction in a variety of taxa, At our study s ite, the beginning of the growing season is controlled by melting of the pr evious winter's snowpack. Despite a trend for warmer spring temperatures th e average date of snowmelt has not changed, perhaps because of the trend fo r increased winter precipitation. This disjunction between phenology at low and high altitudes may create problems for species, such as many birds, th at migrate over altitudinal gradients. We present data indicating that this already may be true for American robins, which are arriving 14 days earlie r than they did in 1981; the interval between arrival date and the first da te of bare ground has grown by 18 days. We also report evidence for an effe ct of climate change on hibernation behavior; yellow-bellied marmots are em erging 38 days earlier than 23 years ago, apparently in response to warmer spring air temperatures. Migrants and hibernators may experience problems a s a consequence of these changes in phenology, which may be exacerbated if climate models are correct in their predictions of increased winter snowfal l in our study area. The trends we report for earlier formation of permanen t snowpack and for a longer period of snow cover also have implications for hibernating species.