Edinburgh Napier University

  Capsule: Over three decades, the garden bird community composition in Edinburgh has changed, probably in response to increased garden bird feeding, and this has increased prey biomass for Eurasian Sparrowhawks Accipiter nisus, coinciding with a significantly higher clutch size of Sparrowhawks over the same period.

Aims: To study the potential impacts of a changing avian prey community on Sparrowhawk breeding success.

Methods: We used a range of datasets to explore how changing trends in garden bird abundance in Edinburgh, UK might impact the diet and breeding success of Sparrowhawks.

Results: Several important Sparrowhawk prey species increased in abundance in Edinburgh between 1998 and 2019, including Woodpigeon Columba palumbus, Goldfinch Carduelis carduelis and House Sparrow Passer domesticus, and others showed decreases, including Starling Sturnus vulgaris, Greenfinch Chloris chloris and Chaffinch Fringilla coelebs. These trends are similar to UK trends for garden bird populations. Sparrowhawk diet shifted towards smaller and larger prey sizes compared to an earlier study conducted between 1986 and 1989. Prey whose contribution to Sparrowhawk diet increased consisted mainly of larger species, such as Woodpigeon, Feral Pigeon Columba livia and Magpie Pica pica. Across the key prey species, there was a significant correlation between trends in prey populations between 1998-2012 and change in the proportion in diet composition, suggesting changing diet reflected a changing prey base. Our results suggest that Sparrowhawk diet has become more diverse, with a more even spread of prey species relative to the earlier study. Sparrowhawk clutch size was significantly higher in 2009-15 compared to 1986-89, with an increase of almost one egg per clutch, but there were no significant differences between these periods in brood size.

Conclusion: Our results are consistent with a bottom-up trophic effect of garden bird community composition on Sparrowhawk clutch size, but not brood size.