INSIGHTS INTO ALTITUDINAL PARTIAL MIGRATION AND THEIR CONSERVATION SIGNIFICANCE
By tracking a population of partially migrating altitudinal migrants for the first time in the Himalayas with the use of state-of-the-art accelerometer informed GPS telemetry, I have successfully illustrated the complex patterns exhibited within a partial altitudinal migration system. The findings from my PhD thesis challenges many of the
assumptions associated with altitudinal migrants and brings new perspectives to our understanding of such systems. In addition, by demonstrating habitat associations, migratory modes and pathways, the thesis contributes towards helping conserve such magnificent migrations which occur across the mountain regions of our planet.
Chapter 1 showed that altitudinal migration systems are complex by documenting migration patterns in a high altitude mountain region for the first time. While I found that more females migrate than males which offer partial support for the body size
(Ketterson & Nolan 1976; Cristol, Baker & Carbone 1999) and the arrival time hypotheses (Ketterson & Nolan 1976; Cristol et al. 1999), I have shown that smaller individuals – both males and females – also ascend to higher elevation sites during winter. As such, I suggest that classical hypotheses (Ketterson & Nolan 1976; Cristol et al. 1999; Chapman et al. 2011) which have been used to explain migration be revisited
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to consider both inter and intra sexual differences. Migrants have also been shown to migrate consistently across years much ahead of snowfall in the region. The idea that extreme weather related events trigger altitudinal migrations (Boyle, Norris & Guglielmo 2010) may therefore be too simplistic an explanation. We also found anecdotal
evidence that individuals can switch strategies from being a migrant to being a resident.
In addition to environmental conditions, I suggest that a host of factors may be at play including parent-offspring learning and density-dependent tradeoffs at breeding sites (Lundberg 1988; Kaitala, Kaitala & Lundberg 1993; Kokko & Lundberg 2001; Taylor &
Norris 2007; Kokko 2011).
Chapter 2 established that altitudinal migration is energetically costly even though they occur over relatively restricted geographic sites. However, there are no tradeoffs related to energy expenditure between migrants and residents. The finding of home range sizes being significantly smaller for migrants perhaps suggests the acquisition of better quality habitats by migrants. However, given that there is no consistency in the altitude and habitats chosen by migrants, it is plausible that individuals end up across a range of habitats with variable intake rates. This is similar for both migrants and residents. As such, at the end of winter, both migrants and residents will fall within a spectrum of energetic balance states dependent on intake rate possibilities. The consequences of where each individual falls on the energetic spectrum will have more significance for males given their need to acquire breeding sites. I suggest that even across small geographic scales, fluctuations in the condition of micro-habitat parameters over time will tip one strategy (i.e. residency vs migratory) over the other differently across years.
In addition to other factors, productivity rates at winter and summer sites can maintain partial migration systems by producing a suite of evolutionary stable states where one
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strategy may not always be the best across time (Berthold 2001; Newton 2008;
Griswold, Taylor & Norris 2011).
Chapter 3 confirmed that altitudinal migrants such as the one presented in this thesis are extremely reliant on forests. Migrants trekked across mountains and halted along slopes using multiple stops during their migration from summer to wintering grounds and back. Migration distances were significant in that the spatial extent of migratory tracks covered almost a third of the protected area’s size (905km2) in which they were tracked. Migratory tracks also demonstrated that corridors in ideals situations would require traversing in random directions across contiguous forest tracts. Importantly, I suggest the inclusion of all habitat configurations (aspects) within protected areas for the effective conservation of altitudinal migratory species in addition to maintaining adequate forest cover given that migrants together with residents relied on forests all year round. I recommend that an assessment of protected areas within mountain regions be conducted and support calls for the maintenance of contiguous forest cover across mountain regions.
My theses is novel in that I have illustrated an as yet un-described migration system and also documented the patterns of an altitudinal migrant in detail for the very first time. I have also tested extant hypotheses and their relevance to the system I observed.
However, the patterns I have documented point towards the need to scale up tracking efforts and concurrently measure explanatory variables to gain a better understanding of such altitudinal partial migration systems.
126 FUTURE DIRECTIONS
I highlight a few areas where major breakthroughs in our understanding of migration biology in general and altitudinal migration in particular will be possible by scaling up and taking on from the work presented in my thesis.
1. Are altitudinal migration systems shaped and maintained by environmental conditions or is there a underlying genetic mechanism (Biebach 1983; Pulido, Berthold & Van Noordwijk 1996) similar to long distance migrants? Amongst other things, repeat strategy (i.e. do migrants always remain migrants) have not been well understood in partial migration systems. Monitoring of individuals across seasons will help clarify this question and contribute significantly in terms of understanding partial migration systems. In addition, there is opportunity to assess the role of parent-offspring learning in shaping migratory systems and assess whether there is indeed a genetic control in such systems. In particular, it would be interesting to test whether altitudinal migrants exhibit migratory restlessness (zugunruhe) (Berthold 1991)?
2. With regards to partial migration, it is still debated as to which strategy (i.e. being a resident or a migrant) offers better fitness outcomes (Gillis et al. 2008; Mackas et al.
2010; Chapman et al. 2011). Monitoring mortality and recruitment rates in parallel with tracking individuals may reveal as yet unknown drivers in such systems and offer essential insights into how such migration systems are maintained over time (Lundberg 1988; Kaitala et al. 1993; Kokko & Lundberg 2001).
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3. In almost all migration systems, it is tacitly assumed that food plays a major role and is indeed a primary cause for migration (Olsson & Greenberg 2006). However, empirical studies have acknowledged that food per se (Boyle 2010) is not an adequate explanation for migrations. Quantifying food availability in addition to conducting food supplementation experiments may help clarify the role and importance of food in shaping migrations.
4. Predation pressure is often invoked as a cause for migration (Boyle 2008; Boyle et al. 2010; Skov et al. 2011, 2013). However, only limited empirical evidence has been provided (Boyle 2008; Hebblewhite & Merrill 2009) and it is not clear on how
predation shapes and influences migratory patterns. In the system we described, given the relatively small geographic scale at which migration occurs, testing the predation hypotheses will be possible through the deployments of camera traps to quantify predation pressure while concurrently tracking predators, migrants and residents.
5. Altitudinal migration is not restricted to birds alone. Since the time of dinosaurs (Fricke, Hencecroth & Hoerner 2011), animals from a range of taxa engage in altitudinal migrations (Shapiro 1973; Mysterud 1999; Rice 2008; Skidmore et al.
2008; Quan et al. 2011; Guan et al. 2013; McGuire & Boyle 2013). An interesting area of pursuit would be to assess whether seasonally migrating montane animals follow similar ques and to examine whether there are general rules which can be applied across taxa in montane environments? Such an endeavour would enable us to predict the consequences of landuse and climate change on altitudinally migrating species across mountainous regions of the world.
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Altitudinal migrations with their occurrence within restricted geographic scales provide a unique opportunity within which to better understand issues within migration science.
Such studies will also hold important lessons for understanding mountain ecosystems and thereby help in their conservation. Given the advances in accelerometer enabled GPS telemetry, my thesis has demonstrated that it is possible to track individuals and illustrate previously unknown patterns over annual cycles. I strongly recommend the setting up of a study system in which possible explanatory variables associated with habitat quality, fecundity, mortality rates, individual condition and predation pressure are measured in parallel with tracking both migrants and residents. I propose that the
system be established to collate data across multiple seasons and years. Such a study system will contribute significantly towards understanding and uncovering both the proximate and ultimate causes and consequences of migrations.
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