Fidelity at the frontier: divorce and dispersal in a newly colonized raptor population R. O. Martin, A. Koeslag, O. Curtis 1 , A. Amar * Percy FitzPatrick Institute of African Ornithology, DST/NRF Centre of Excellence, University of Cape Town, Rondebosch, South Africa article info Article history: Received 11 November 2013 Initial acceptance 13 December 2013 Final acceptance 3 April 2014 Published online MS. number: 13-00950R Keywords: Accipiter melanoleucus better option hypothesis black sparrowhawk breeding dispersal colonization incompatibility hypothesis partner fidelity population growth range expansion range margin Divorce and breeding dispersal are key life history parameters that can be influenced by, and in turn have an influence on, the structure of populations. Variation in these parameters in small populations can potentially play an important role in the colonization of new areas, yet to date there has been little empirical investigation of this process. We studied the circumstances surrounding divorce in a newly established population of black sparrowhawks, Accipiter melanoleucus, on the Cape Peninsula, South Africa over an 11-year period between 2001 and 2012. Divorce was more likely following breeding failure and individuals that divorced and dispersed improved their subsequent breeding success. Territory quality had no influence on the frequency of divorce, and dispersing individuals did not move to terri- tories of higher quality. During the study period the population size increased approximately three-fold, but because the range expanded, nest density did not increase significantly. In the first half of the study (2001e2006), divorce rates were low (4%) compared with rates previously reported for raptors and other birds. In the latter half (2007e2012) divorce rates were 14%. Although there was weak support for a difference in divorce rates between these two periods there was no evidence that divorce rates varied with population size or nesting density. Our results suggest that adaptive hypotheses (‘better option’ or ‘incompatibility’) best explain patterns of divorce in this expanding population and that potential feedbacks between divorce and population processes were unlikely to have played an important role during the growth of this recently established population. Ó 2014 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Mating systems can be considered as dynamic arrangements in which individuals make decisions over whom they partner with depending on the prevailing circumstances (Emlen & Oring, 1977; Reynolds, 1996). Changes in the population size and density can determine the availability of potential partners or territories and strongly influence mating decisions (Kokko & Rankin, 2006). In turn, these decisions may have implications for demographic pro- cesses such as population growth. Such dynamic two-way in- teractions can be important in small populations, potentially driving extinctions through Allee effects, or facilitating the coloni- zation of new areas (Bessa-Gomes et al., 2003; Stephens & Sutherland, 1999). For example, the population growth of reintro- duced Critically Endangered Seychelles magpie robins, Copsychus seychellarum, was initially rapid, but as populations increased, higher rates of competition for territories suppressed offspring production (Lopez-Sepulcre, Norris, & Kokko, 2009). Similarly, if divorce leads to lost breeding opportunities for divorce victims (Moody, Wilhelm, Cameron-MacMillan, Walsh, & Storey, 2005), lowering of divorce rates at low population density could mean improved population growth, counteracting potential Allee effects (Kokko & Rankin, 2006). In range-expanding populations colonizing new areas, in- dividuals are frequently exposed to novel environmental conditions and changing demographic pressures. In monogamous territorial species, an individual’s decision of whether to stay with the same partner, divorce, disperse or attempt to force its way into a breeding partnership depends on the relative costs and benefits of the available options (Coulson, 1972; Ens, Safriel, Harris, 1993). Among other factors these costs and benefits will depend on the avail- ability and quality of alternative partners, the availability and quality of alternative territories and the availability of information with which to assess these alternatives (Choudhury, 1995). Varying population size, breeding density, territory quality and familiarity of individuals with their environment can therefore have a strong influence on the breeding decisions of a population colonizing a new area. * Correspondence: A. Amar, Percy FitzPatrick Institute of African Ornithology, DST/NRF Centre of Excellence, University of Cape Town, Rondebosch 7701, South Africa. E-mail address: arjun.amar@uct.ac.za (A. Amar). 1 Present address: Overberg Lowlands Conservation Trust, Napier, South Africa. Contents lists available at ScienceDirect Animal Behaviour journal homepage: www.elsevier.com/locate/anbehav http://dx.doi.org/10.1016/j.anbehav.2014.04.018 0003-3472/Ó 2014 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Animal Behaviour 93 (2014) 59e68