Plastic territoriality in group-living chestnut-crowned babblers: roles of resource value, holding potential and predation risk Enrico Sorato a, b, * , Philippa R. Gullett a, c , Matthew J. S. Creasey d , Simon C. Griffith a, e , Andrew F. Russell d a Department of Biological Sciences, Macquarie University, Sydney, NSW, Australia b Station d’Ecologie Exp erimentale du CNRS, Moulis, France c Department of Animal and Plant Sciences, University of Sheffield, Sheffield, U.K. d Centre for Ecology and Conservation, College of Life and Environmental Sciences, University of Exeter, Penryn Campus, Penryn, U.K. e School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW, Australia article info Article history: Received 23 May 2014 Initial acceptance 26 June 2014 Final acceptance 14 November 2014 Available online 24 January 2015 MS. number: 14-00423R Keywords: cooperative breeding habitat quality habitat saturation playback experiment sociality The factors selecting for territoriality and their relative importance are poorly resolved. Theoretical models predict that territoriality will be selected when resources of intermediate abundance are distributed variably and predictably in time and space, but can be selected against if the resource-holding potential of individuals is low or the risk of predation is high. Here we used a model averaging approach in a mixed modelling framework to analyse 5 years of observational and experimental data collected on group responses to actual and perceived intruders in the cooperatively breeding chestnut-crowned babbler, Pomatostomus ruficeps, in order to provide a rare test of the relative importance of resource value, resource-holding potential and predation risk in territorial behaviour. We found that babblers were highly plastic in their responses to actual and simulated intruders: on average, approaches occurred on 55% of occasions, and aggression ensued in 55% of approaches (observational and experimental re- sults combined). Whether or not babbler groups approached, and if so were aggressive towards, actual or simulated intrusions was explained by time of day, location, group sizes, predator encounter rate and habitat characteristics, but not by reproductive status. Consideration of each of these effects regarding the three hypotheses above suggested comparable roles of group competitive advantage and predation risk on approach probability, whereas ensuing aggression was mostly explained by correlates of resource value. Our study provides compelling evidence to suggest that the risk of predation can affect the incidence of territorial and agonistic behaviour between social groups of animals by moderating the effects of resource value and group competitiveness, and might partly explain the high plasticity in group responses to intrusions. © 2015 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Territoriality has defining effects on population phenotype and structure, but underlying selection pressures remain contentious (Brown, 1969; Clutton-Brock, Green, Hiraiwa-Hasegawa, & Albon, 1988; Gordon, 1997; Newton, 1992; Packer et al., 2005). Early the- ory proposed that the incidence of territoriality can be understood in economic terms, with individuals being territorial when the payoffs of defending an exclusive area exceed the sum of costs involved (Brown, 1964; Davies, 1980; Davies & Houston, 1981; Kodric-Brown & Brown, 1978). Classic studies of territorial behav- iour in wintering golden-winged sunbirds, Drepanorhynchus reichenowi (Gill & Wolf, 1975) and pied wagtails, Motacilla alba (Davies, 1976) are testament to the fruits of this approach. Never- theless, for year-round residents with contiguous territories in which the benefits and costs of territoriality can be varied (Adams, 2001; Lima, 1984; Maher & Lott, 2000), identifying the salient correlates of territorial behaviour and integrating them into a unifying economic currency becomes increasingly challenging, and risks misinterpretation of the relative importance of constituent parameters (Maher & Lott, 2000). The most commonly reported associates of territorial behaviour are spatiotemporal aspects of resource distribution (Maher & Lott, 2000). Chief among these is a common positive association be- tween the spatiotemporal predictability of resources (particularly food) and the incidence of territorial behaviour (Maher & Lott, * Correspondence: E. Sorato, Department of Biological Sciences, Macquarie University, Sydney NSW 2109, Australia. E-mail address: enrico.srt@gmail.com (E. Sorato). Contents lists available at ScienceDirect Animal Behaviour journal homepage: www.elsevier.com/locate/anbehav http://dx.doi.org/10.1016/j.anbehav.2014.12.012 0003-3472/© 2015 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Animal Behaviour 101 (2015) 155e168