Nest predation and habitat change interact to influence Siberian jay numbers So ¨nke Eggers, Michael Griesser, Tommy Andersson and Jan Ekman Eggers, S., Griesser, M., Andersson, T. and Ekman, J. 2005. Nest predation and habitat change interact to influence Siberian jay numbers. / Oikos 111: 150  /158. We studied Siberian jays, breeding in northern Sweden, to examine the potential for interactions between nest predation and reduced vegetation heterogeneity around nest sites to cause a decrease in jay numbers. Parent behaviour and nests are highly cryptic in the species. Our 12-year data showed, however, that nests had a probabilityof only 0.46 to be successful and produce at least one nestling. Nest predation was intense and a main cause of nest failure. All predators that could be identified were visually oriented hunters, mostly other corvids able to colonize taiga forest only close to human settlements. Consistent with the idea that predators used visual cues, nest predation increased with parental activity, which suggests that predators used parental provisioning trips to locate nests. Furthermore, a reduction in daily nest survival rates with decreasing amount of nesting cover was more pronounced in areas with high corvid activity as predicted when cover mediates the hunting efficiency of visual oriented predators. Declining temperatures interacted with the effects of habitat characteristics to further reduce daily nest survival rates suggesting that parents were not able to increase nest visitation rates to satisfy the higher energy demands of their nestlings without endangering the nest. Our results identify a mechanism through which predation and human-induced reduction in nesting cover on a larger scale may interact to cause a reduction in Siberian jay numbers larger than expected from habitat loss alone. S. Eggers, M. Griesser and J. Ekman, Evolutionary Biology Centre, Dept of Population Biology, Uppsala Univ., Norbyva ¨gen 18D, SE-752 36 Uppsala, Sweden. Present address for SE: Dept of Zoology, Stockholm Univ., SE-10691 Stockholm, Sweden. (sonke.eggers@zoologi.su.se).  / T. Andersson, Dept of Physics, Uppsala Univ., La ¨ geruddsva ¨ gen 1, SE-751 21 Uppsala, Sweden. Habitat loss and direct shortage of food supplies may often provide the ultimate cause for both short- and long term declines of animal numbers (Grant 1986, Hanski et al. 1991). Yet, predation can hold the population size of prey species well below the level that resources would permit and may even cause their longterm decline and demise (Potts 1986, Marcstro ¨m et al. 1988, Thirgood et al. 2000). In general, however, the question of whether predation impacts and habitat change interact to influ- ence population dynamics has been rarely addressed (Evans 2004). Human-induced habitat changes can affect bird po- pulations in many ways (Gates and Gysel 1978, Andre ´n 1992). One idea is that reductions in the quantity and quality of nesting cover increase the exposure of avian nests to visually hunting predators (Potts 1986, Martin and Roper 1988, Martin 1992a, Roper and Goldstein 1997, Ekman et al. 2001). Factors that are likely to modify the role of habitat structure for the risk of nest predation include the nest type involved, the abundance and searching behaviour of predators and the parents’ capacity to behave cryptically, to deter or to distract Accepted 7 March 2005 Copyright # OIKOS 2005 ISSN 0030-1299 OIKOS 111: 150 /158, 2005 150 OIKOS 111:1 (2005)