Oikos 120: 1761–1763, 2011 doi: 10.1111/j.1600-0706.2011.20070.x © 2011 he Authors. Oikos © 2011 Nordic Society Oikos Subject Editor: Tim Benton. Accepted 7 July 2011 Partial migration: an introduction Ben B. Chapman, Christer Brönmark, Jan-Åke Nilsson and Lars-Anders Hansson B. B. Chapman (ben.chapman@biol.lu.se), C. Brönmark, J.-Å. Nilsson and L.-A. Hansson, Dept of Biology, Lund Univ., Ecology Building, ES-223 62 Lund, Sweden. Movement is a fundamental and ubiquitous feature of animals, and the movement of individual organisms is integral for many vital ecological and evolutionary processes (Nathan et al. 2008). Animal migration is one of nature’s most spectacular forms of animal movement and has a long and illuminating scientific history, with exciting new discoveries reported each year (Klaassen et al. 2011). Migration is a diverse phenomenon, and can be categorised into a multitude of forms. he most common type of migration is known as ‘partial migration’, which is characterised by within-population variation in migratory tendency such that just a fraction of the population migrates (Lack 1943). Hence partially migratory populations consist of both migratory and resident individuals. It is taxonomically very widespread in the animal kingdom, and has been documented in many species, from white bream to wildebeest (Chapman et al. 2011a). he study of partial migration is important as it can shed light upon migratory adaptations, the maintenance of life-history polymorphisms in animal populations, and more generally on the evolution of migratory behaviour itself. Historically the phenomenon of partial migration has received relatively little research attention compared to other areas of migration biology. his is perhaps surprising, given the potential ecological importance of partial migration (Brodersen et al. 2008) and its prevalence in nature. One might speculate that the erstwhile negligence of this field has been primarily due to the logistical difficulties inherent in tracking individual animals through space and time, differentiating between migrant and resident individuals over long periods, and documenting sufficient variation to allow for hypothesis testing (sample sizes can be extremely small in descriptive migration studies). his notion is supported by the historical research trajectory of partial migration: in the early days of this field most empirical work was strictly descriptive and speculative (Lack 1943). his was followed by a pulse of powerful theoretical work which laid a framework for testing hypotheses with field data (Cohen 1967, Lundberg 1987, Kaitala et al. 1993). he analytic empirical work in many taxa lagged behind until technological advances (and the concomitant reduction in the price and size of tracking devices and increase in computing power) have allowed researchers to track large numbers of migratory animals with high resolution across time and space and analyse individual variation in their seasonal patterns of movement. hese technological solutions have facilitated a burst of high quality empirical papers over the past decade, testing hypotheses of partial migration in a diverse array of animals, from ungulates to crustaceans to fish (Hebblewhite and Merrill 2007, Hansson and Hylander 2009, Chapman et al. 2011b). Indeed, the future of such studies looks bright, with the continual advent of increasingly powerful technologies such as quantum dots to track even extremely small animals as they (partially) migrate (Lard et al. 2010). In addition, our theoretical understanding of partial migration has developed from simple graphical representations (Lundberg 1987) to more complex evolutionary invasion analysis (Kokko and Lundberg 2001, Kokko 2007, Griswold et al. 2010). In recognition of the continuing theoretical development and recent empirical renaissance of this field we convened an international symposium in ‘he Ecology and Evolution of Partial Migration’ at Lund University in autumn 2010, funded by the CAnMove research constellation. CAnMove is a centre of excellence for trans-disciplinary research on the causes and consequences of animal movements, funded by a Linnaeus grant from the Swedish Research Council and Lund University (⬍ http://canmove.lu.se/ ⬎). Our aim was to bring together a disparate group of theoreticians, field biologists and animal behaviourists to review the current state of the field, to present original research and to discuss future directions in partial migration research. In this Oikos thematic we have curated a selection of original research articles in the resurgent field of partial migration, many of which were discussed and presented at the symposium. Each article is at the cutting edge of partial migration research, and was chosen not just as representative of the high quality research being carried out in this area, but also to highlight new directions and to sketch out the future of this fascinating area of migration biology. he contributions cover a diverse range of taxa and highlight the breadth of methodologies and techniques employed to address questions in partial migration research. he structure of the thematic reflects the wide scope of its content and we open this thematic with a review of what is known about the ecology and evolution of partial migration, and suggest future avenues for research focus (Chapman et al. 2011a). In this review we evaluate the 1761 proximate and ultimate causes of partial migration, and also discuss the potential consequences of partial migration in an ecological and evolutionary context. he next series of papers focus upon migration at an individual level. Pulido (2011) proposes an environmental threshold model to explain the proximate control mechanisms that determine whether an individual is a migrant or a resident. He extends the threshold model of migration to include the influence of environmental variation, and discusses the model in light of the general evolution of migratory behaviour. In the third paper, Nilsson et al. (2011) highlight the utility of studying migratory adaptations in partially migratory species by comparing physiological adaptations in migrant and resident blue tits Cyanistes caerulus. hey show that residents have a higher basal metabolic rate than migrants, which may reflect either an adaptation to winter conditions on the resident grounds or alternatively be a corollary of dominance status and play a role in intraspecific competition for resources during autumn. he next study analyses an impressive individualbased data set of roe deer Capreolus capreolus partial migration from five different European countries (Cagnacci et al. 2011). his study shows interesting interactions between different environmental variables and migration, and importantly highlights that partial migration is a behavioural continuum with a spectrum of individual movement categories within it. he next three papers focus upon evaluating the ecological drivers of partial migration, and analyse data with an ambitious cross-population or community scope. Boyle (2011) presents an innovative community-level test of the limited foraging opportunity hypothesis in Neotropical birds. She shows that patterns of altitudinal partial migration correlate with extreme wet season weather events such as tropical storms, which supports the hypothesis that limited foraging opportunities drives the migration of metabolically challenged individuals to habitats with more benign weather conditions across a range of Neotropical bird species. Analysing data from multiple populations of red deer Cervus elaphus, Mysterud et al. (2011) show that many ecological factors shape the dynamics of partial migration in northern ungulates, including competition avoidance of high densities during the winter and social fencing (i.e. negative density-dependent migration) during the summer. Kokko (2011) also focuses upon the role of intraspecific competition in facultative partial migration. She explores the implications of relaxing some of the assumptions made by previous theoretical work into the evolution of partial migration, and shows that the dynamics of territory acquisition are critically important in shaping patterns of partial migration. Either dominant or subdominant individuals can migrate dependent upon the strength of the prior residency effect. An interesting additional outcome of this study is that it shows that individual adaptations for intraspecific competition can lead to population declines. he following paper shifts from a focus upon causes of partial migration and rather addresses the ecological consequences. Brodersen et al. (2011) analyse a longitudinal data set of partially migratory roach Rutilus rutilus which migrate out of shallow lakes, and show that partial migration can have significant top–down consequences at an ecosystem level. hey report that the intensity of migration influences plankton 1762 spring dynamics by shifting the timing of the zooplankton peak. We then introduce a series of papers which investigate how anthropogenic influences can shape the dynamics of partial migration. Griswold et al. (2011) present a theoretical population model that shows how the population size of migrants and residents are dependent on one another as their dynamics are coupled via density-dependent effects. hey also highlight how their model can be used to test hypotheses about the influence of environmental change on partially migratory populations. Hebblewhite et al. (2011) analyse demographic data from partially migratory elk Cervus elaphus and show that anthropogenic factors can impact migrants and resident populations in different ways by altering the costs and benefits of migration/residency. heir study highlights the importance of understanding migratory dynamics when considering the conservation of partially migratory species. he final paper of our thematic broadens previous notions of partial migration to include scenarios where animals migrate to breed, but not every year, hence creating patterns of partial migration. Shaw and Levin (2011) present the first theoretical consideration of this kind of partial migration, which is widespread and has been described in many species. hey assess under what conditions individuals should skip migration in constant and stochastic environments. Partial migration research has been recognised as being at the frontier of the new discipline of movement ecology (Sekercioglu 2010), and this is exemplified by the diverse and exciting research highlighted in this thematic. It is our belief that the recent resurgence of interest in partial migration will continue to grow, and that the future will bring many more innovative and important research findings. We hope that this thematic can contribute to this promising future by stimulating inventive and ambitious research programs to further understand the enduring puzzle of migratory dimorphism in animals. Acknowledgements – We thank CAnMove for funding the symposium in partial migration that sparked this thematic edition of Oikos; Helena Osvath and Maria Eng-Johnsson for logistical support and organisational assistance; all symposium participants for thought provoking discussions and for making the symposium a stimulating and exciting event; Johan Algren, Sophia Engel and Martin Stålhammar for good grilled food, and Markus Ljungkvist for juggling enlightenment. BBC received support from the Centre for Animal Movement Research (CAnMove) financed by a Linnaeus grant (349-2007-8690) from the Swedish Research Council and Lund Univ.; CB, LAH and JAN acknowledge VR for funding. We would also like to express our great appreciation for the considerable efforts provided by the reviewers of all the thematic contributions. Last (but certainly not least!) we thank Tim Benton and Linus Svensson for their support, encouragement and patience in putting together this thematic. References Boyle, W. A. 2011. Short-distance partial migration of Neotropical birds: a community-level test of the foraging limitation hypothesis. – Oikos 120: 1803–1816. Brodersen, J. et al. 2008. Ecosystem effects of partial fish migration in lakes. – Oikos 117: 40–44. Brodersen, J. et al. 2011. Interplay between temperature, fish partial migration and trophic dynamics. – Oikos 120: 1838–1846. Cagnacci, F. et al. 2011. Partial migration in roe deer: migratory and resident tactics are end points of a behavioural gradient determined by ecological factors. – Oikos 120: 1790–1802. Chapman, B. B. et al. 2011a. he ecology and evolution of partial migration. – Oikos 120: 1764–1775. Chapman, B. B. et al. 2011b. To boldy go: individual differences in boldness influence migratory tendency. – Ecol. Lett. 14: 871–876. Cohen, P. 1967. Optimization of seasonal migratory behaviour. – Am. Nat. 101: 5–17. Griswold, C. K. et al. 2010. he evolution of migration in a seasonal environment. – Proc. R. Soc. B 277: 2711–2720. Griswold, C. K. et al. 2011. he equilibrium population size of a partially migratory population and its response to environmental change. – Oikos 120: 1847–1859. Hansson, L.-A. and Hylander, S. 2009. Size-structured risk assessments govern Daphnia migration. – Proc. R. Soc. B 276: 331–336. Hebblewhite, M. and Merrill, E. H. 2007. Multiscale wolf predation risk for elk: does migration reduce risk? – Oecologia 152: 377–387. Hebblewhite, M. and Merrill, E. H. 2011. Demographic balancing of migrant and resident elk in a partially migratory population through forage–predation tradeoffs. – Oikos 120: 1860–1870. Kaitala, A. et al. 1993. A theory of partial migration. – Am. Nat. 142: 59–81. Klaassen, R. H. G. et al. 2011. Great flights by great snipes: long and fast non-stop migration over benign habitats. – Biol. Lett. in press. Kokko, H. 2007. Modelling for field biologists and other interesting people. – Cambridge Univ. Press. Kokko, H. 2011. Directions in modelling partial migration: how adaptation can cause a population decline and why the rules of territory acquisition matter. – Oikos 120: 1826–1837. Kokko, H. and Lundberg, P. 2001. Dispersal, migration and offspring retention in saturated habitats. – Am. Nat. 157: 188–202. Lack, D. 1943. he problem of partial migration. – Br. Birds 37: 122–130. Lard, M. et al. 2010. Tracking the small with the smallest – using nanotechnology in tracking zooplankton. – PLoS One 5: e13516. Lundberg, P. 1987. Partial bird migration and evolutionarily stable strategies. – J. heor. Biol. 125: 351–340. Mysterud, A. et al. 2011. Partial migration in expanding red deer populations at northern latitudes – a role for density dependence. – Oikos 120: 1817–1825. Nathan, R. et al. 2008. A movement ecology paradigm for unifying organismal movement research. – Proc. Natl Acad. Sci. USA 105: 19052–19059. Nilsson, A. L. K. et al. 2011. Basal metabolic rate and thermoregulatory capacity among migratory and resident blue tits. – Oikos 120: 1784–1789. Pulido, F. 2011. Evolutionary genetics of partial migration – the threshold model of migration revis(it)ed. – Oikos 120: 1776–1783. Sekercioglu, C. 2010. Partial migration in birds: the frontier of movement ecology. – J. Anim. Ecol. 79: 933–936. Shaw, A. K. and Levin, S. A. 2011. To breed or not to breed: a model of partial migration. – Oikos 120: 1871–1879. 1763