Extreme migration and the annual cycle : individual strategies in New Zealand Bar-tailed Godwits : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Ecology at Massey University, Palmerston North, New Zealand
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Long-distance migration places severe constraints on the annual cycles of birds, as they balance the energetic and scheduling requirements of breeding, moult, pre-migratory fuelling, and the journey itself. The most extreme migrations, traversing vast, inhospitable areas of the globe in protracted non-stop flights, may push birds to the limits of their capabilities, and would be expected to tolerate little variation in performance. Despite this, Bar-tailed Godwits Limosa lapponia baueri, which are among the world’s greatest endurance migrants, embark on northward migration from New Zealand across a month-long period, and individuals are quite faithful to their particular schedules. Godwits are highly sexually dimorphic in plumage and body size, and there is additionally substantial individual variation within each sex in both traits. These patterns demonstrate a surprising diversity of strategies within a system that should contain little room for error. In this thesis, I sought to identify the roots and consequences of both persistent and ephemeral individual differences in migration and moult of New Zealand Bar-tailed Godwits, and to identify constraints and potential bottlenecks in their annual cycle. To do this, I combined a fine-resolution multi-year focus on individuals and an entire annual-cycle perspective, both of which have generally been impossible in studies of long-distance migratory birds. At a single non-breeding site, I closely monitored moult and migration of individual Bar-tailed Godwits for three non-breeding seasons, and linked these with events outside of New Zealand by tracking a subset of the same individuals on their complete migrations to Alaska breeding grounds and back. I supplemented this by travelling to Alaska myself and describing how godwits are distributed by size and plumage across their vast breeding range. I found that most of the variation among individual Bar-tailed Godwits was linked to where they nested in Alaska: within each sex, northerly breeders were smaller, had more extensive breeding plumage, and migrated later on both northbound and southbound migrations. The differences in migration timing can be explained by variation in when tundra breeding sites become snow-free and available across a latitudinal gradient, but reasons for geographic differences in plumage and size are less clear. Variation in breeding plumage was associated with different strategies for scheduling moult, both in New Zealand and during northbound migratory stopover in the Yellow Sea. Individual godwits were extraordinarily consistent between years in their timing of departure from New Zealand, and most ‘off-schedule’ departures were attributable to birds avoiding unfavourable winds for migration. Surprisingly, timing of arrival in New Zealand after the longest recorded non-stop flight did not appear to influence a godwit’s ability to prepare for its next migration, as timing of subsequent migratory departure and extent of breeding plumage on departure were both unaffected and very consistent. Across the entire year, scheduling of events became more precisely timed as the breeding season approached, but movements were generally much more tightly scheduled than moults. These findings show that Bar-tailed Godwits adopt and enact an array of individualised strategies within an apparently constrained system. The inter-relationships among events in different parts of the globe show that an individual-based, full annual-cycle perspective is required to understand patterns in any particular season. The consistent manner in which godwits conduct their annual routines, while still demonstrating flexibility to address unforeseen circumstances, challenges us to reconsider the view of extreme long-distance migrants as organisms operating at the limits of their capabilities.
Chapters 1-6 have been published: Chapter 2. Conklin, J.R., Battley, P.F., Potter, M.A., & Fox, J.W. (2010).Breeding latitude drives individual schedules in a trans-hemispheric migrant bird. Nature Communications, 1(67). doi:10.1038/ncommms1072 Chapter 3. Conklin, J.R., Battley, P.F., Potter, M.A. & Ruthrauff, D.R. (2011). Geographic variation in morphology of Alaska-breeding bar-tailed godwits (Limosa lapponica) is not maintained on their non-breeding grounds in New Zealand. The Auk, 128(2):363-373. doi:10.1525/auk.2011.10231 Chapter 4. Conklin, J.R., & Battley, P.F. (2011). Impacts of wind on individual migration schedules of New Zealand Bar-tailed Godwits. Behavioral Ecology, 22(4): 854-861. doi:10.1093/beheco/arr054 Chapter 5. Conklin, J.R., & Battley, P.F. (2011). Contour-feather moult of Bar-tailed Godwits (Limosa lapponica baueri) in New Zealand and the northern hemisphere reveals multiple strategies by sex and breeding region. Emu, 111: 330-340. doi:10.1071/MU11011 Chapter 6. Conklin, J.R., & Battley, P.F. (2012). Carry-over effects and compensation : late arrival on non-breeding grounds affects wing moult but not plumage or schedules of departing Bar-tailed Godwits. Journal of Avian Biology. doi:10.1111/j.1600-048X.2012.05606.x
Bar-tailed godwit, Limosa lapponia baueri, New Zealand, Bird migration, Long distance migration, Bird moulting, Migratory behaviour