Flexible response to short‐term weather in a cold‐adapted songbird

Abstract

To improve survival during winter, temperate species use a variety of behavioural and physiological adaptations. Among songbirds, the maintenance of lipid reserves is a widely‐used strategy to cope with the severity of winter; however, little is known regarding how multiple synchronously acting environmental mechanisms work together to drive these effects. In a context where temperate winter conditions are becoming more variable, it is important to widen our understanding regarding the flexible adaptations that may allow wintering species to adjust to projected climate change. Using a long‐term dataset collected across multiple wintering populations (7 years; 8 locations), we analyzed the effects of daily variation in weather (e.g. temperature, snowfall) on the variation in energy reserves (i.e. fat stores) of wintering snow buntings Plectrophenax nivalis. Our results support the prediction that birds carry more reserves to increase the safety margin against starvation when conditions are energy‐demanding and access to food is unpredictable (i.e. colder, snowier conditions). Birds responded to daily changes in weather by increasing their reserves as conditions deteriorated, with maximal temperatures and snow depth being the most important predictors of fattening decisions. We also found that females consistently exhibited higher fat reserves than males relative to their body size, suggesting that differential physiological adaptations among sexes or social dominance may play an additional role in explaining variation in energy reserves across individuals in this species. Overall, our findings increase knowledge on phenotypic adjustments used by species wintering in temperate zones to match variation in their environment.

Tracking Landscape-Scale Movements of Snow Buntings and Weather-Driven Changes in Flock Composition During the Temperate Winter

“…The long-term banding data analyses revealed maximum temperatures and snow depth were the most important weather variables correlated with changes in flock composition by sex and size. These results are consistent with our earlier study showing that these variables were also central in informing daily fattening patterns in Snow Buntings (Laplante et al, 2019). Snow depth may rank foremost, considering that snow buntings are granivorous ground-feeders which can have more difficulty accessing resources in areas of high snow depth, and record-high numbers of Snow Buntings have been observed during warmer (less snowy) winters in Central Europe (Orlowski and Gebski, 2007).…”

“…Movements may also be influenced by male dominance over food resources, where females may be forced to move away when weather is severe because they are simply unable to access food and accumulate the required fat store to survive cold nights. Laplante et al (2019) reported that for their body size, females were carrying more fat than males. In Snow Buntings wintering in Scotland, females were more likely to move when flock sizes were larger, presumably because they were excluded by dominant individuals when intraspecific competition increased (Smith and Metcalfe, 1997a).…”

“…We obtained snow depth data from the National Snow and Ice Data Center (NSIDC) (Brasnett, 1999;Brown and Brasnett, 2010) using a grid cell (24 km x 24 km) that included the banding site in Fergus. In total, we extracted eight daily weather variables, which based on previous work on Snow Buntings and others, are predicted to have an influence on snow bunting behavior (Orlowski and Gebski, 2007;Macdonald et al, 2016;Laplante et al, 2019): minimum temperature ( • C), mean temperature ( • C), maximum temperature ( • C), snow depth (cm), total snowfall (cm), absolute humidity (g/m 3 ), maximum wind gust (km/h), and cloud cover (0-10).…”

“…Some migratory songbirds may hedge their bets by remaining as far north as possible (in otherwise marginal habitats), and only moving south in the case of extreme disruption of resources (i.e., "fugitive" migration patterns) (Terrill and Ohmart, 1984;Watts et al, 2018). Unfortunately, little is known about exactly how and why variability in weather parameters (i.e., temperature, snow cover, humidity, wind) trigger movement in wintering songbirds Laplante et al, 2019).…”

“…Snow Buntings also exhibit male-biased sexual size dimorphism and previous work has suggested that larger individuals may have a thermoregulatory advantage in colder weather, losing less heat per unit mass than small individuals Laplante et al, 2019). Harsh winter weather could thus have a different impact on the energy budgets and movement decisions of males and females, due to differential thermoregulatory constraints among the sexes, potentially also explaining why females carry larger energy reserves per unit size than males Laplante et al, 2019). Furthermore, since buntings are highly gregarious in winter and exhibit dominance hierarchies within flocks (Smith and Metcalfe, 1997a,b), social hierarchies could also generate differential pressure to move in interaction with weather (Ketterson and Nolan, 1979).…”