Multiple drivers of spring migration timing for red deer over the past 16 years in northern Europe

The timing of migration is fundamental for species exploiting seasonally
variable environments. For ungulates, earlier spring migration is expected
with earlier vegetation green-up. However, other drivers, such as access
to agricultural farmland and variations in local conditions, are also
known to affect migration. We investigated the timing of spring migration
for 96 male and 201 female red deer (Cervus elaphus) using a long-term
dataset (2005-2020). Overall, the timing of migration was mainly
characterised by large individual variability between and within years
(95% range from April 6th to June 18th). The spring migration timing was,
as expected, later with colder winter and spring conditions (NAO winter
and April indices) and later peak vegetation green-up (NDVI), with a 5-day
delay in green-up causing a migration delay of 1.2 days. The timing was
also influenced by local conditions in summer and winter home ranges. Red
deer with higher access to farmland and more variable topography (hence
variable plant phenology) in winter delayed migration. Similarly,
individuals with higher elevation summer ranges (with delayed onset of
plant growth) also delayed migration. Our analyses highlight that the
timing of red deer migration is formed by multiple drivers affecting
foraging conditions in the landscape, indicative of considerable
phenotypic plasticity.