OIKOS 93: 388 – 400. Copenhagen 2001 Are goose nesting success and lemming cycles linked? Interplay between nest density and predators Joe ¨l Be ˆty, Gilles Gauthier, Jean-Franc ¸ois Giroux and Erkki Korpima ¨ki Be ˆty, J., Gauthier, G., Giroux, J.-F. and Korpima ¨ki, E. 2001. Are goose nesting success and lemming cycles linked? Interplay between nest density and predators. – Oikos 93: 388–400. The suggested link between lemming cycles and reproductive success of arctic birds is caused by potential effects of varying predation pressure (the Alternative Prey Hypothesis, APH) and protective association with birds of prey (the Nesting Associ- ation Hypothesis, NAH). We used data collected over two complete lemming cycles to investigate how fluctuations in lemming density were associated with nesting success of greater snow geese (Anser caerulescens atlanticus ) in the Canadian High Arctic. We tested predictions of the APH and NAH for geese breeding at low and high densities. Goose nesting success varied from 22% to 91% between years and the main egg predator was the arctic fox (Alopex lagopus ). Nesting associations with snowy owls (Nyctea scandiaca ) were observed but only during peak lemming years for geese nesting at low density. Goose nesting success declined as distance from owls increased and reached a plateau at 550 m. Artificial nest experiments indicated that owls can exclude predators from the vicinity of their nests and thus reduce goose egg predation rate. Annual nest failure rate was negatively associated with rodent abundance and was generally highest in low lemming years. This relationship was present even after excluding goose nests under the protective influence of owls. However, nest failure was inversely density-dependent at high breeding density. Thus, annual variations in nest density influenced the synchrony between lemming cycles and oscillations in nesting success. Our results suggest that APH is the main mechanism linking lemming cycles and goose nesting success and that nesting associations during peak lemming years (NAH) can enhance this positive link at the local level. The study also shows that breeding strategies used by birds (the alterna- tive prey) could affect the synchrony between oscillations in avian reproductive success and rodent cycles. J. Be ˆty and G. Gauthier (correspondence), De ´partement de biologie and Centre d’e ´tudes nordiques, Paillon Vachon, Uniersite ´ Laal, Sainte -Foy, QC, Canada G1K 7P4 (gilles.gauthier@bio.ulaal.ca).– J.-F. Giroux, De ´partement des sciences biologiques, Uniersite ´ du Que ´bec a ` Montre ´al, C.P. 8888, Succursale Centre -ille, Montre ´al, QC, Canada, H3C 3P8.– E. Korpima ¨ki, Section of Ecology, Dept of Biology, Uni. of Turku, FIN-20014 Turku, Finland. In arctic tundra and northern Europe, vole (Clethriono - mys and Microtus spp.) and lemming (Lemmus and Dicrostonyx spp.) populations vary enormously in size over time and these fluctuations follow fairly regular cycles of three to five years (Krebs 1964, Hanski and Korpima ¨ki 1995). Marked between-year variations in the reproductive success of several species of arctic ground-nesting birds have also been observed and related to cyclical changes in the abundance of small mammals (e.g. Summers 1986, Martin and Baird 1988, van Impe 1996). Two non-exclusive hypotheses have been proposed to explain this potential link. The Alternative Prey Hypothesis (APH) states that the link is caused by a varying annual predation pres- sure on alternative prey resulting from the functional and numerical responses of predators to rodent densi- Accepted 2 February 2001 Copyright © OIKOS 2001 ISSN 0030-1299 Printed in Ireland – all rights reserved OIKOS 93:3 (2001) 388