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Decline in territory size and fecundity as a response to carrying capacity in an endangered songbird


Density-dependent processes are fundamental mechanisms for the regulation of populations. Ecological theories differ in their predictions on whether increasing population density leads to individual adjustments of survival and reproductive output or to dominance and monopolization of resources. Here, we use a natural experiment to examine which factors limit population growth in the only remaining population of the endangered pale-headed brush finch (Atlapetes pallidiceps). For three distinct phases (a phase of population suppression, 2001–2002; expansion due to conservation management, 2003–2008; and equilibrium phase, 2009–2014), we estimated demographic parameters with an integrated population model using population size, the proportion of successfully breeding pairs and their productivity, territory size, and mark-recapture data of adult birds. A low proportion of successful breeders due to brood parasitism (0.42, 95% credible interval 0.26–0.59) limited population growth before 2003; subsequent culling of the brood parasite resulted in a two-fold increase of the proportion of successful breeders during the ‘expansion phase’. When the population approached the carrying capacity of its habitat, territory size declined by more than 50% and fecundity declined from 1.9 (1.54–2.27) to 1.3 (1.12–1.53) chicks per breeding pair, but the proportion of successful breeders remained constant (expansion phase: 0.85; 0.76–0.93; equilibrium phase: 0.86; 0.79–0.92). This study demonstrates that limiting resources can lead to individual adjustments instead of despotic behavior, and the individual reduction of reproductive output at high population densities is consistent with the slow life-history of many tropical species.

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We thank the Deutsche Ornithologische Gesellschaft (DO-G), the Eva-Mayr-STIHL-Stiftung, the Wissenschaftliche Gesellschaft Freiburg, Müller-Fahnenberg-Stiftung, and the Swedens Club 300 Bird Protection for financial support. We thank Fundacion Jocotoco for cooperation and permission to work in their reserve Yunguilla. We further thank P. A. Carrasco Ugalde, C. Hermes, and M. Hoffmann for their help in mist-netting. Permissions to conduct field work (032-DPA-MA-2012 and 038-DPA-MA-2013) were granted by the Ministerio del Ambiente and by the Ministerio de Agricultura, Ganaderia, Acuacultura y Pesca, Ecuador.

Author contribution statement

SAH, GS and HMS originated and developed the idea for the study. SAH, SO and MJ conducted the field work. SO developed the integrated population model. SAH and SO analyzed the data. SAH and SO wrote the manuscript, GS and HMS contributed to writing the manuscript

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Correspondence to Stefanie A. Hartmann.

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Communicated by Hannu J. Ylonen.

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Hartmann, S.A., Oppel, S., Segelbacher, G. et al. Decline in territory size and fecundity as a response to carrying capacity in an endangered songbird. Oecologia 183, 597–606 (2017). https://doi.org/10.1007/s00442-016-3763-6

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  • Carrying capacity
  • Cowbird parasitism
  • Density-dependence
  • Integrated population model
  • Population regulation