Data from: Winter chilling speeds spring development of temperate butterflies

Understanding and predicting phenology has become more important with
ongoing climate change and has brought about great research efforts in the
recent decades. The majority of studies examining spring phenology of
insects have focussed on the effects of spring temperatures alone. Here we
use citizen-collected observation data to show that winter cold duration,
in addition to spring temperature, can affect the spring emergence of
butterflies. Using spatial mixed models, we disentangle the effects of
climate variables and reveal impacts of both spring and winter conditions
for five butterfly species that overwinter as pupae across the UK, with
data from 1976 to 2013 and one butterfly species in Sweden, with data from
2001 to 2013. Warmer springs lead to earlier emergence in all species and
milder winters lead to statistically significant delays in three of the
five investigated species. We also find that the delaying effect of winter
warmth has become more pronounced in the last decade, during which time
winter durations have become shorter. For one of the studied species,
Anthocharis cardamines (orange tip butterfly), we also make use of
parameters determined from previous experiments on pupal development to
model the spring phenology. Using daily temperatures in the UK and Sweden,
we show that recent variation in spring temperature corresponds to 10–15
day changes in emergence time over UK and Sweden, whereas variation in
winter duration corresponds to 20 days variation in the south of the UK
versus only 3 days in the south of Sweden. In summary, we show that short
winters delay phenology. The effect is most prominent in areas with
particularly mild winters, emphasising the importance of winter for the
response of ectothermic animals to climate change. With climate change,
these effects may become even stronger and apply also at higher latitudes.