Negotiating the northern edge of a nonbreeding distribution: a case of winter mortality in Eurasian Spoonbills

Abstract During a cold spell from 6 to 14 February 2021, we monitored the fate of Eurasian Spoonbills Platalea leucorodia leucorodia on Schouwen-Duiveland, province of Zeeland, The Netherlands. On 6 February 2021, average daily air temperature dropped to below 0 °C, and for the next 8 days frost was maintained, with thaw arriving on 15 February. As the wind picked up as well, (human) wind chill temperatures dropped to below – 10 °C during this entire period. The victims at the Levensstrijd wetland near Zierikzee, fully ice covered within a day after the drop in temperatures, were all juvenile (i.e. born in 2020). They stood waiting before dying on 12–13 February (six were found and collected). They remained at Levensstrijd despite one marked individual among them having previously been at sites which kept having open water during the cold spell. This included the canals near Burgh-Haamstede (ca. 8 km away) where 26 birds (mostly adult) were foraging during the cold spell. Nevertheless, two spoonbills were found dead here too, one of which was an adult. The eight collected carcasses were stored frozen for subsequent analysis of body composition. The winter victims weighed 71% (4 males)–76% (4 females) of the asymptotic body mass predicted for growing juveniles. The dissected specimens showed no fat, their masses (and those of the organs and muscles as well) tightly correlating with body size. The mass fractions of the different body parts of the winter-starved spoonbills (compared to ‘normal’) were high for lungs (i.e. no loss), medium for leg muscles (a loss of 20%) and low for the flight muscles and the internal organs heart, stomach, intestines and liver (losses of 40–65%). We propose that the cold spell victims had shifted from using fat to using protein as their reserve fuel, at which point air temperatures of -5 °C and wind speeds of 8–10 m/s must have induced hypothermia and death. It did not kill birds in nearby flocks with access to open water, birds which must have had a better body condition and could still rely on fat as fuel. This study demonstrates the susceptibility of especially young spoonbills to food becoming locally unavailable during frost events during which surface waters freeze over. We are puzzled by the lack of movements to nearby places with open water and food when Levensstrijd froze solid, as the birds were capable of flight even on the day before being found dead. Does lethargy and the strong cold winds explain it, or do social inhibitions (by the victims) and despotic behaviours (by the surviving adults at the open water site) play a role? Although it is likely that juvenile, rather than adult, spoonbills will be the first to winter on a shifting northern edge of the nonbreeding distribution, these youngsters are also most at risk when cold spells hit.