Food availability modifies abundance of small birds in cold winters, but body condition is worse under high predation risk
​A pygmy owl. The predator used in this study. Pygmy owls often hunt during the day and are efficient predators of small passerines
​Predation risk did not have clear effects on abundance of tit species but influenced food consumption and, together with food supplementation, affected the deposition of subcutaneous fat in great tits. The detrimental effects of predation risk on fat deposits may reduce over-winter survival, but the costs imposed by pygmy owl risk were compensated when food was supplemented. In small bird species living in harsh environment, the starvation-predation trade-off appeared thus to be biased towards avoidance of starvation, at the cost of increasing predation risk.

Our results allow a better understanding on how these environmental factors affect animal populations and may help in planning efficient conservation measures for endangered species. Our results suggest that during winter abundance of small passerine birds, in northern coniferous forests, is mainly determined by food availability. A simple but effective conservation action could thus be to provide food supplements for birds, which could potentially induce exciting increases in population densities of decreasing songbird species in heavily managed North European boreal forests.

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-Chiara Morosinotto