Caught out in the cold: Mallard survival decreased during an extreme climatic event

Extreme climatic events (ECEs) can have profound impacts on individual
fitness, affecting survival directly or indirectly. Late winter ECEs may
be especially detrimental to fitness due to limited food resources and
increased energetic requirements during this time. A polar vortex
disruption ECE descended upon the mid-continental United States during
7–20 February 2021 with temperatures as low as −29ºC in areas concurrent
with ongoing research on mallard (Anas platyrhynchos) movement ecology and
survival in Arkansas, Louisiana, and Tennessee, USA spanning winters
2019–2022. Therefore, we opportunistically evaluated the effects of
individual characteristics and latitude on daily survival during the ECE.
We extended the survival analysis into March to test for lasting effects
of the ECE on survival. We tracked 181 GPS-marked mallards during February
2020, 256 in February 2021, and 324 in February 2022. We documented 22
mortalities during the February 2021 ECE (i.e., 9%), but only 6
mortalities during February 2020 (i.e., 2%) and 2022 (i.e., 1%) when
conditions were average. February survival (e.g., 28-day survival) during
the ECE was 0.908 (85% CI = 0.879–0.937) but was 0.982 (85% CI = 0.973 –
0.991) during the two non-ECE Februaries. The ECE effect on survival was
isolated to February and did not affect March survival. Mallards were 5.4
times more likely to die during the ECE in 2021 compared to non-ECE
Februaries. Although large-bodied waterfowl appear cold-tolerant and less
sensitive to polar vortex disruptions compared to smaller-bodied
passerines, direct mortalities can occur if conditions are severe enough
and persist, highlighting the need to consider the influence of ECEs on
common, seemingly robust species in future global climate change
scenarios.