1. Population dynamics of specialist predators are tightly linked to their main prey and can in simple food webs lead to complex predator-prey interactions (e.g. predator-prey cycles). However, the strength of these interactions may be affected by the availability of alternative prey if it appears in high numbers and the predator diet is sufficiently flexible.

2. A prominent example of a specialist predator-prey interaction is the interaction between gyrfalcon and ptarmigan known from simple food webs with few alternative preys. Here we use a 23-year long time series from a more complex food web in northern Fennoscandia to investigate the relative roles of main (ptarmigan) and cyclically fluctuating alternative prey (lemmings) in driving gyrfalcon nesting territory occupancy and production of young.

3. We find that nesting territory occupancy dynamics are dependent on both main and alternative prey, while fledgling production within occupied territories is independent of prey abundance dynamics. Population density of willow ptarmigan had the expected direct and delayed effect on gyrfalcon territory occupancy dynamics, while one high-amplitude lemming population peak year in the time series boosted colonization of nesting territories.

4. This study demonstrates that the population dynamics of alternative prey can play an important role for specialist predators. While it might be uplifting that the gyrfalcons, being a regionally red listed species, seem to be able to consume alternative prey, both ptarmigan and lemmings are negatively impacted by ongoing climate change. Hence, the population status of gyrfalcon should still be a conservation concern.