Combined food and predator effects on songbird nest survival and annual reproductive success: results from a bi-factorial experiment
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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.
KeywordsFood supplementation Nest predation Predator pressure Song sparrows
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.
- Grzybowski JA, Pease CM (2005) Renesting determines seasonal fecundity in songbirds: what do we know? What should we assume? Auk 122:280–291Google Scholar
- Hodges KE, Krebs CJ, Hik DS, Stefan CI, Gillis EA, Doyle CE (2001) Snowshoe hare demography. In: Krebs CJ, Boutin S, Boonstra R (eds) Ecosystem dynamics of the boreal forest. Oxford University Press, Oxford, pp 141–178Google Scholar
- Krebs CJ (1999) Ecological methodology, 2nd edn. Benjamin/Cummings, Menlo Park, Calif.Google Scholar
- Lack D (1947) The significance of clutch size. Ibis 89:302–352Google Scholar
- Lack D (1954) The natural regulation of animal numbers. Oxford University Press, OxfordGoogle Scholar
- Newton I (1993) Predation and limitation of bird numbers. Curr Ornithol 11:143–198Google Scholar
- Newton I (1998) Population limitation in birds. Academic, LondonGoogle Scholar
- Skutch AF (1949) Do tropical birds raise as many young as they can nourish? Ibis 91:430–455Google Scholar
- Smith JNM, Taitt MJ, Rogers CM, Arcese P, Keller LF, Cassidy ALEV, Hochachka WM (1996) A metapopulation approach to the population biology of the song sparrow Melospiza melodia. Ibis 138:120–128Google Scholar
- Zanette L, Clinchy M, Smith JNM (2006) Food and predators affect egg production in song sparrows. Ecology (in press)Google Scholar