Abstract
Species distributions shift northwards due to climate change, but the ecological mechanisms allowing range expansions are not fully understood. Most studies have concentrated on breeding seasons, but winter warming may also be important. Wintering distributions are restricted by food availability and temperature, which may also interact. Foraging in cold conditions requires adaptations as individuals have to be efficient in foraging, while staying warm and vigilant for predators. When the ambient temperature declines, foraging rates should be reduced due to increased time spent on warming behaviours. In addition, predator vigilance should decline, because more time has to be invested in foraging. Cold weather should limit northward expanding southern species in particular, while northern species should perform better in cold conditions. We tested this by studying temperature responses (between 0 and − 35 °C) among wintering birds at feeders. We compared foraging behaviours of two northward expanding southern species, the great tit (Parus major) and the blue tit (Cyanistes caeruleus) to a northern species, the willow tit (Poecile montanus). Foraging rate and vigilance decreased, and warming behaviour increased when temperatures declined. Importantly, the performance in these traits was poorer in the southern species compared to the willow tit. Furthermore, the response to decreasing temperatures in foraging rates and warming behaviour was stronger in the great tits than willow tits. As the winters become warmer, these mechanisms should increase wintering success of southern species wintering at high latitudes, and lead to higher survival, increased population growth, and consequent range expansion.
Highlights
Climate change shifts species distributions polewards in most taxa (Thomas and Lennon 1999; Parmesan and Yohe 2003; Brommer et al 2012; Lehikoinen and Virkkala 2016)
Overall foraging rates decreased with decreasing temperatures, and great tits and blue tits had lower foraging rates than willow tits (Fig. 1, Table 1)
Warming behaviour increased in response to decreasing temperatures, and warming was more frequent in great tits and blue tits than in willow tits (Table 2; Fig. 2)
Summary
Climate change shifts species distributions polewards in most taxa (Thomas and Lennon 1999; Parmesan and Yohe 2003; Brommer et al 2012; Lehikoinen and Virkkala 2016). Most studies have concentrated on breeding season phenological consequences of climate change (e.g., Visser et al 1998; Vatka et al 2011, 2014), while wintering ecology is less examined despite shortening winters. Warming of winters may be an important factor behind range expansion, because surviving the winter should become easier further north (Maclean et al 2008; Zuckerberg et al 2011; Lehikoinen et al 2013; Fraixedas et al 2015), but the ecological mechanisms allowing range expansions are not fully understood (Gaston 2009). At the same time, when food availability is reduced, low temperatures require higher food consumption due to increased energy demands for heat production. Successful wintering in cold conditions where temperatures can decline below − 30 °C requires morphological, physiological, and behavioural adaptations
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