red deer GPS data
The Forage Maturation Hypothesis (FMH) states that herbivores migrate along a phenological gradient of plant development in order to maximize energy intake. Despite strong support for the FMH, the actual relationship between plant phenology and ungulate movement has remained enigmatic. We linked plant phenology (MODIS-NDVI) and space use of 167 migratory and 78 resident red deer (Cervus elaphus) using a space-time-time matrix of "springness", defined as the instantaneous rate of green-up (IRG). Consistent with the FMH, migrants experienced substantially greater access to early plant phenology than residents. Deer were also more likely to migrate in areas where migration led to greater gains in springness. Rather than "surfing the green wave" during migration, migratory red deer moved rapidly from the winter to the summer range, thereby "jumping the green wave". However, migrants, and to a lesser degree residents, did track phenological green-up through parts of the growing season by making smaller-scale adjustments in habitat use. We conclude that, whereas in some systems migration itself is a way to surf the green wave, in others it may simply be a means to re-connect with phenological spring at the summer range.