Life balance - to eat or be eaten?

Submitted by editor on 24 November 2016.Get the paper!

Food availability and predation risk can have drastic impacts on animal behaviour and populations. The trade-off between foraging and predator avoidance is fundamental for survival of animals and will strongly modify 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, particularly in harsh winter conditions in boreal forests. We studied the effects of food supplementation and natural predation risk imposed by pygmy owls on the abundance and fat reserves of tit species in coniferous forest patches in western Finland, from January to March in 2012 and again in 2013. The pygmy owl is the worst avian enemy of small bird species over-wintering in boreal forests. This is shown, for example, by the very strong mobbing behaviour exhibit by resident birds towards pygmy owls, particularly in winter.

Food supplementation increased the number of tit individuals present in a given forest patch, whereas the level of pygmy owl risk had no clear impact on the abundance of tit species (mainly great, willow, crested, blue and coal tits). The stronger impact of food supply respect to owl 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 for small tit species.

Predation risk did not have obvious effects on abundance of tit species but influenced food consumption and, together with food supplementation, affected the deposition of subcutaneous fat in great tits. 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 trade-off faced by great tits in winter may thus be mediated through variation in body fat reserves. In small bird species living in harsh environment, this trade-off appeared thus to be biased towards avoidance of starvation, at the cost of increasing predation risk.

Our results, combined with previous studies on the impact of food-supplementation and predation risk, allow us 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. This in turn means that a simple but effective conservation action could be to provide food supplements for birds. The sole addition of food supplements, even without further laborious measures such as predator removal, could potentially induce exciting increases in population densities of decreasing songbird species in heavily managed North European boreal forests.

The authors through Chiara Morosinotto


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