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.

食物可获得性与捕食风险均会对动物行为及种群动态产生强烈影响。觅食与反捕食之间的权衡对动物存活至关重要，且会显著影响个体体重——充足的脂肪储备虽有助于降低饥饿风险，却可能提升被捕食的概率。然而，同时探究食物与捕食风险交互效应的双因子实验仍较为稀缺。
本研究于2012年和2013年的1月至3月间，在北欧寒温带针叶林内开展野外实验，探究食物补充以及侏儒猫头鹰（Glaucidium passerinum）施加的自然捕食风险，对山雀类种群丰度及脂肪储备的影响。结果表明，食物补充提升了特定林块内的山雀个体数量，而捕食风险水平对山雀种群丰度并无明显作用。相较于捕食风险，食物供应的影响更为显著，这可能源于北欧严冬的恶劣环境：当地气温持续低于冰点，且每日仅能提供5至7小时的觅食光照时长。
捕食风险虽未对山雀种群丰度产生明显影响，却会改变其食物摄取量；且捕食风险与食物补充共同作用，会影响大山雀（Parus major）的皮下脂肪沉积。较高的猫头鹰捕食风险会对大山雀的体脂储备产生不利影响，进而降低其越冬存活率；但当食物得到补充时，侏儒猫头鹰捕食风险带来的生存成本会得到补偿。
因此，冬季大山雀所面临的饥饿-捕食权衡，或可通过体脂储备的变化进行调节。对于栖息在恶劣环境中的小型鸟类而言，这种权衡往往倾向于规避饥饿，却以提升被捕食风险为代价。