Abstract
Food and predators have traditionally been viewed as mutually exclusive alternatives when considering factors affecting animal populations. This has led to long controversies such as whether annual reproductive success in songbirds is primarily a function of food-restricted production or predator-induced loss. Recent studies on both birds and mammals suggest many of these controversies may be resolved by considering the combined effects of food and predators. We conducted a 2×2 manipulative food addition plus natural predator reduction experiment on song sparrows (Melospiza melodia) over three consecutive breeding seasons. Food and predators together affected partial clutch or brood loss, nest survival (total clutch or brood loss) and annual reproductive success. When combined, our two treatments reduced partial losses by more than expected if the effects of food and predators were independent and additive. Food and predators also interacted in their effects on nest survival since food addition significantly reduced the rate of nest predation. While annual reproductive success was highly correlated with nest predation (r 2=0.71) the strength of this relationship was reinforced by the indirect effects of food addition on nest predation. A stepwise multiple regression showed that the residual variation in annual reproductive success was explained by food effects on the total number of eggs laid over the season and the combined effects of food and predators on partial losses noted above. We conclude that annual reproductive success in song sparrows is a function of both food-restricted production and predator-induced loss and indirect food and predator effects on both clutch and brood loss. We highlight the parallels between our results and those from a comparable bi-factorial experiment on mammals because we suspect combined food and predator effects are likely the norm in both birds and mammals.
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Acknowledgements
We thank Harry van Oort, Anne Duncan-Rastogi, Jeannie Trudeau, Josh Malt, Chris de Ruyck, Andrew Davis, Lionel Leston and Nathalie Denis for assistance in the field; BC Parks, the Saanich Municipality and private landowners for access to the sites; Sharon Hartwell and the Rithet’s Bog Conservation Society and Beryl Clinchy for support; and the Chow Barn for accepting our feed deliveries. Tim Karels, Anne Duncan-Rastogi, Bethany Kempster and an anonymous reviewer provided many helpful comments on the manuscript. This research was funded by the Natural Sciences and Engineering Research Council of Canada. All experiments complied with the laws of Canada.
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Zanette, L., Clinchy, M. & Smith, J.N.M. Combined food and predator effects on songbird nest survival and annual reproductive success: results from a bi-factorial experiment. Oecologia 147, 632–640 (2006). https://doi.org/10.1007/s00442-005-0330-y
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DOI: https://doi.org/10.1007/s00442-005-0330-y