Data from: Food supplementation and predation risk in harsh climate: interactive effects on abundance and body condition of tit species

Food availability and predation risk can have drastic impacts on animal behaviour and populations. The tradeoff between foraging and predator avoidance is crucial for animal survival and will strongly affect individual body mass, since large fat reserves are beneficial to reduce starvation but may increase predation risk. However, two-factor experiments simultaneously investigating the interactive effects of food and predation risk, are still rare. We studied the effects of food supplementation and natural predation risk imposed by pygmy owls Glaucidium passerinum on the abundance and fat reserves of tit species in boreal forests of north Europe, from January to March in 2012 and in 2013. Food supplementation increased the number of individuals present in a given forest patch, whereas the level of predation risk had no clear impact on the abundance of tit species. The stronger impact of food supply respect to predation risk could be the consequence of the harsh winter conditions in north Europe, with constant below-zero temperatures and only few (5-7 h) daylight hours available for foraging. Predation risk did not have obvious effects on tit abundance but influenced food consumption and, together with food supplementation, affected the deposition of subcutaneous fat in great tits Parus major. High owl predation risk had detrimental effects on body fat reserves, which may reduce over-winter survival, but the costs imposed by pygmy owl risk were compensated when food was supplemented. The starvation–predation tradeoff faced by great tits in winter may thus be mediated through variation in body fat reserves. In small species living in harsh environment, this tradeoff appeared thus to be biased towards avoidance of starvation, at the cost of increasing predation risk.

食物可获得性（food availability）与捕食风险（predation risk）会对动物的行为及种群数量产生显著影响。觅食与反捕食之间的权衡对动物存活至关重要，且会强烈影响个体体重：较多的脂肪储备（fat reserves）有助于降低饥饿风险，但同时可能提升被捕食概率。然而，同时探究食物与捕食风险交互效应的双因子实验（two-factor experiments）仍较为少见。我们于2012年及2013年的1-3月，在北欧北方针叶林（boreal forests）中开展研究，探究食物补充（food supplementation）以及侏儒猫头鹰（Pygmy Owl，*Glaucidium passerinum*）带来的自然捕食风险（natural predation risk），对山雀类（tit species）种群丰度（abundance）及脂肪储备的影响。食物补充提升了特定林块（forest patch）内的山雀个体数量，而捕食风险水平对山雀类的种群丰度无明显影响。相较于捕食风险，食物供应的影响更为显著，这可能源于北欧严酷的冬季环境：气温持续低于零度，且可供觅食的日照时长仅为5-7小时。捕食风险对山雀种群丰度无显著作用，但会影响其食物摄入量（food consumption）；且捕食风险与食物补充共同作用，会影响大山雀（Great Tit，*Parus major*）的皮下脂肪沉积（subcutaneous fat deposition）。较高的猫头鹰捕食风险会对个体脂肪储备产生不利影响，进而降低越冬存活率，但当食物得到补充时，侏儒猫头鹰捕食风险带来的生存代价会得到抵消。因此，大山雀在冬季面临的饥饿-捕食权衡（starvation–predation tradeoff），可能通过个体脂肪储备的变化得以调节。对于栖息在严酷环境中的小型鸟类而言，这种权衡似乎更倾向于规避饥饿，代价则是提升了被捕食风险。