, Volume 149, Issue 3, pp 543–552 | Cite as

Density-dependent regulation of population size in colonial breeders: Allee and buffer effects in the migratory Montagu’s harrier

  • Alvaro Soutullo
  • Rubén Limiñana
  • Vicente Urios
  • Martín Surroca
  • Jennifer A. Gill
Behavioural Ecology


Expanding populations offer an opportunity to uncover the processes driving spatial variation in distribution and abundance. Individual settlement decisions will be influenced by the availability and relative quality of patches, and by how these respond to changes in conspecific density. For example, conspecific presence can alter patch suitability through reductions in resource availability or territorial exclusion, leading to buffer effect patterns of disproportionate population expansion into poorer quality areas. However, conspecific presence can also enhance patch suitability through Allee effect processes, such as transmission of information about resources or improved predator detection and deterrence. Here, we explore the factors underlying the settlement pattern of a growing population of Montagu’s harriers (Circus pygargus) in Spain. The population increased exponentially between 1981 and 2001, but stabilised between 2001 and 2004. This population increase occurred alongside a remarkable spatial expansion, with novel site use occurring prior to maximum densities in occupied sites being reached. However, no temporal trends in fecundity were observed and, within sites, average fecundity did not decline with increasing density. Across the population, variance in productivity did increase with population size, suggesting a complex pattern of density-dependent costs and benefits. We suggest that both Allee and buffer effects are operating in this system, with the benefits of conspecific presence counteracting density-dependent declines in resource availability or quality.


Allee effect Buffer effect Density dependence Ideal despotic distribution Ideal free distribution 



We are very grateful to Aeropuerto de Castellon for support and funding. We are thankful to Beatriz Arroyo, Luis Cadahía and Carlos Passos for insightful discussions of these ideas. A great part of the data presented here come from the monitoring and conservation programme of Montagu’s harrier in Castellon conducted by the Generalitat Valenciana since 1988. These data have been provided by D.G. de Gestión del Medio Natural (Consellería de Territorio y Vivienda). Thanks are due to Pepe Durall, Sara Ferreras, Juan Jiménez, Stephan Miralles, Juanjo Palomo, Pepe Rovira, Miguel Ángel Solís and Ramón Prades for their help. We are particularly grateful to Liberto Villaroya and Anita. R.L. has a graduate fellowship from the Generalitat Valenciana. This paper is part of his PhD thesis at the Universidad de Alicante.


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Copyright information

© Springer-Verlag 2006

Authors and Affiliations

  • Alvaro Soutullo
    • 1
  • Rubén Limiñana
    • 1
  • Vicente Urios
    • 1
  • Martín Surroca
    • 2
  • Jennifer A. Gill
    • 3
  1. 1.Estación Biológica Terra Natura (CIBIO-Fundación Terra Natura) Universidad de AlicanteAlicanteSpain
  2. 2.Centro de recuperación de Fauna “Forn del Vidre”Generalitat ValencianaCastellonSpain
  3. 3.School of Biological SciencesUniversity of East AngliaNorwichUK

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