Predation risk and competition in mixed species tit flocks

Abstract

In coniferous forests of Northern Europe, tits (Parus spp.) and the goldcrest (Regulus regulus) exploit nonrenewable resources m their group territories during winter. Results of many studies have indicated that interspecific competition restricts the use of foraging sites in mixed species winter flocks of tits. There was a high correlation between food abundance and bird density over different parts of spruce suggesting that the availability of food influences the distribution of foraging sites of tits and goldcrest. However, foraging site distributions cannot be explained solely by food abundance. In outer tree parts there were fewer birds than would be expected from arthropod biomass. A possible explanation is that the risk of predation by pygmy owls (Glaucidium passerinum) is high in the outer tree parts reducing the net benefit of foraging in the food-richest tree parts. The risk of predation by: the pygmy owl is higher for subdominant species foraging in the exterior tree parts, i.e. the coal tit (P. ater) and the golacrest. The dominant species, the crested tit (P. cristatus) and the willow tit (P. montanus), which forage on inner tree parts, were killed less frequently by the pygmy owl. The risk of predation by pygmy owls varies greatly with the availability of voles as the preferred prey. Both willow and crested tits changed to forage in inner parts of coniferous trees after the vole populations had crashed and when pygmy owls started to hunt tits. Additionally both in pine and birch willow tits moved towards the tree tops, the safest sites of those trees, under high predation risk. However, there were individual differences in the response to the change in the predation risk level. Both female crested tits and adult willow tits moved from outer parts of spruce towards the tree trunk with increased risk of predation. However, male crested tits, which are highest in social rank in the flock, exhibited no response to the pygmy owl, but they were rather safe from predation in any case. It seems that both interspecific competition and site-dependent risk of predation govern the ways in which these four bird species select their foraging sites in spruce. Additional food has been shown to increase winter survival of tits. Tits regularly hoard enormous amounts of seeds mainly during the autumn presumably to be used during the winter; Willow and crested tits tended to avoid to hoard in the locations in which the other species forages, suggesting that the risk of interspecific kleptoparasitism influences the selection of hoarding sites. An aviary experiment demonstrated that the cacher willow tit has a higher recovery rate for its own caches than for caches made by other individuals. Moreover, the individuals of another species included in the experiment, great tits, had lowest recovery rate of caches. It seems that food storing is adaptive to the cachers because they have the best recovery rate for their own stored seeds. However, the results indicate that there remained a high risk to lose the caches to conspecific individuals and low probability to lose the caches to individuals of other species.