, Volume 166, Issue 1, pp 79–90 | Cite as

Coping with uncertainty: breeding adjustments to an unpredictable environment in an opportunistic raptor

  • Fabrizio SergioEmail author
  • J. Blas
  • L. López
  • A. Tanferna
  • R. Díaz-Delgado
  • J. A. Donázar
  • F. Hiraldo
Population ecology - Original Paper


No environment is truly constant in time. As a result, animals have evolved multiple adjustments to cope with such fluctuations. However, the allocation of effort to costly activities that imply long-term commitments, such as breeding, may be extremely challenging when future resources change constantly and unpredictably, a context that has received little investigation. To fill this gap, we studied the breeding response by a wetland-dependent raptor, the black kite Milvus migrans, to within and between-years fluctuations in resource availability (inundation levels). The breeding performance of the population was decomposed into reproductive components expressed in a sequence of successive tasks along the breeding cycle (e.g. timing of laying, clutch size, hatching success, brood reduction). Variation in each component was related to resource levels observed at different key dates of the season in order to test whether and when population-level reproduction was adjusted to available resources. Along a 22-year time-series, inundation levels fluctuated unpredictably within and among years, and mostly affected the later components of kites’ reproduction, such as hatching success and the incidence of brood reduction, which were the main determinants of the population yearly breeding output. Results were consistent with multiple adjustments to cope with uncertainty. As the season progressed and resources became easier to assess, a bet-hedging waiting strategy based on a conservatively small, invariant and asynchronous clutch gave way to real-time resource-tracking mechanisms mediated by progressive adjustments to current prey availability, so that population-level breeding rates were determined and tuned to resources rather late in the season. Such adjustments were the likely outcome of the interaction between parental tactics and environmental constraints. Behavioural flexibility, such as dietary opportunism, probably promoted further resistance to resource oscillations. Given that all ecosystems show some degree of unpredictability, resource-tracking adjustments, such as the ones depicted here, are likely to be commonplace in most communities.


Brood reduction Environmental uncertainty Hydrology Inundation-rates Resource fluctuations Water-levels 



We thank F.G. Vilches, R. Baos, S. Cabezas, M.G. Forero, G. García, L. García, J. Giralt, M. Guerrero and A. Sánchez for help in the field. We thank H. Pietiäinen, J. Valkama and an anonymous reviewer for comments on a previous draft of the manuscript. Data on rabbits road-transects were kindly provided by the “Equipo de Seguimiento de Procesos Naturales” of the Estación Biológica de Doñana. Part of this study was funded by the research projects CGL2008-01781 of the Ministerio de Ciencia e Innovación, JA-58 of the Consejería de Medio Ambiente de la Junta de Andalucía and by the Excellence Project RNM 1790 and RNM 03822 of the Junta de Andalucía to F.S.

Supplementary material

442_2010_1795_MOESM1_ESM.pdf (57 kb)
Supplementary material 1 (PDF 57 kb)
442_2010_1795_MOESM2_ESM.pdf (284 kb)
Supplementary material 2 (PDF 284 kb)


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Copyright information

© Springer-Verlag 2010

Authors and Affiliations

  • Fabrizio Sergio
    • 1
    Email author
  • J. Blas
    • 1
  • L. López
    • 1
  • A. Tanferna
    • 1
    • 2
  • R. Díaz-Delgado
    • 1
  • J. A. Donázar
    • 1
  • F. Hiraldo
    • 1
  1. 1.Department of Conservation BiologyEstación Biológica de Doñana, CSICSevillaSpain
  2. 2.DISUAN, Department of Human, Environmental and Natural SciencesUniversity of UrbinoUrbinoItaly

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