Group dynamics, critical group size effects and extinction risk

Understanding population dynamics is crucial for effective conservation biology. In many cases breeding is limited by high density, but in social species the opposite is true, exposing small groups to high extinction risk. However, analyses of population dynamics in social species is rare, limiting our ability to effectively conserve such species. Professor Ridley uses long-term observations of habituated groups of cooperatively breeding birds to ask important questions regarding population dynamics and extinction risk

It has become increasingly obvious that just as reproduction and survival can be constrained when population density is high, in some cases reproduction and survival can also decline dramatically at low density or group size. It is clearly essential to understand the behavioural basis of these latter phenomena, and how the interactions between individuals and groups within populations shape the risk of local extinction. Some cooperative breeders, where individuals live in groups to rear offspring, represent a highly suitable model for the study of these effects. However, gathering appropriate information is constrained by logistical trade-offs because understanding population dynamics and extinction risk require long time-series of data on the fate of both individuals and the groups in which they live. The often subtle behavioural interactions (eg reproductive conflict leading to delayed reproduction) that determine whether groups expand or shrink in size require intensive and intricate study.
Combining the two approaches has proved almost impossible, since long-term studies can generally only be conducted with some sacrifice of detail.

The research Professor Ridley pursues overcomes these difficulties: he has invested enormous effort over 11 years to establish a completely habituated population of pied babblers (Turdoides bicolor), which allows more detailed characterisation of behaviour than has been achieved for any other free-living cooperatively breeding species. These birds forage and interact oblivious to his presence, allowing him to collect behavioural information to an exceptionally detailed degree. In addition, an entirely unique and extremely valuable aspect of the population is that he can weigh the birds on a daily basis, providing unprecedented information regarding the costs and benefits of behaviours both during the short- and long-term.

Professor Ridley uses this unique population and dataset to:

• Examine the social dynamics that set limits to group size
• Determine the behavioural underpinnings of reproductive failure and group extinction
• And examine the extent to which reproductive failure and extinction risk are exacerbated or moderated by climate events


  • Professor Hanna Kokko, University of Zurich
  • Associate Professor Arpat Ozgul, University of Zurich