Capsule: A decline in polygyny and lower secondary female breeding success have reduced male productivity and most likely led to the population decline. Aims: To examine whether changes in demographic parameters during a population decline fit with a hypothesis that there has been a reduction in food supplies. Methods: We examined changes in the number of young, numbers of successful broods and the size of these broods, produced on west Mainland, Orkney between 1953 and 2000 from ringing returns and original nest records. We also examined the changes in polygyny levels between 1967-74, 1976-81 and 1998-2000. Using more intensive data from 1980-81 and 1998-2000, we examined whether there were any changes in various breeding parameters of primary (monogamous or alpha) females or secondary (beta) females. Results: Numbers of young and broods produced have declined dramatically in the west Mainland of Orkney since the end of the 1970s. In the 1970s, an average of 60 chicks fledged each year, whereas an average of only 16 chicks fledged in the 1990s. However, over this same period, the average brood size of successful nests did not change. The percentage of males breeding polygynously decreased from around 75% during 1967-81 to only 17% during 1998-2000, and the breeding success of secondary females also decreased. As a result, productivity of males on Orkney was significantly lower in 1998-2000 than in 1980-81. Conclusions: The Hen Harrier population has declined dramatically on Orkney since the end of the 1970s. This was due to a decrease in polygynous breeding and a reduction in the breeding success of secondary females. Changes to demographic parameters were consistent with a hypothesis that the decline was an effect of reduced food availability during the early part of the breeding season during recent years.