Abstract
The degree to which extrinsic factors influence migration chronology in North American waterfowl has not been quantified, particularly for dabbling ducks. Previous studies have examined waterfowl migration using various methods, however, quantitative approaches to define avian migration chronology over broad spatio-temporal scales are limited, and the implications for using different approaches have not been assessed. We used movement data from 19 female adult mallards (Anas platyrhynchos) equipped with solar-powered global positioning system satellite transmitters to evaluate two individual level approaches for quantifying migration chronology. The first approach defined migration based on individual movements among geopolitical boundaries (state, provincial, international), whereas the second method modeled net displacement as a function of time using nonlinear models. Differences in migration chronologies identified by each of the approaches were examined with analysis of variance. The geopolitical method identified mean autumn migration midpoints at 15 November 2010 and 13 November 2011, whereas the net displacement method identified midpoints at 15 November 2010 and 14 November 2011. The mean midpoints for spring migration were 3 April 2011 and 20 March 2012 using the geopolitical method and 31 March 2011 and 22 March 2012 using the net displacement method. The duration, initiation date, midpoint, and termination date for both autumn and spring migration did not differ between the two individual level approaches. Although we did not detect differences in migration parameters between the different approaches, the net displacement metric offers broad potential to address questions in movement ecology for migrating species. Ultimately, an objective definition of migration chronology will allow researchers to obtain a comprehensive understanding of the extrinsic factors that drive migration at the individual and population levels. As a result, targeted conservation plans can be developed to support planning for habitat management and evaluation of long-term climate effects.
Highlights
Migration is a fundamental aspect of natural history in many species and has profound implications to vital rates and individual fitness [1,2]
Geopolitical Boundaries Based on the geopolitical method for delineating migratory events, autumn migration in 2010 (n = 8) had a mean midpoint of 15 November 2010 (SD = 6.0 days) and mean duration of 24.5 days (SD = 16.1) (Figure 2a)
We compared individual migration chronology estimates from a quantitative technique based on a distance metric (ND) with estimates from a geopolitical method that examined bird movement relative to state, provincial, and international boundaries [23,24,25]
Summary
Migration is a fundamental aspect of natural history in many species and has profound implications to vital rates and individual fitness [1,2]. Definitions of migration differ among taxa, the term generally represents individual movements that collectively facilitate a population level outcome [8]. In birds, migration is considered an annual event with distinct seasonal movements between breeding and non-breeding areas [9]. North American waterfowl exhibit some of the most visible seasonal migrations, moving between breeding and wintering areas in the spring and autumn [13]. In addition to a limited understanding of extrinsic migration drivers, waterfowl movement and seasonal ranges are increasingly affected by anthropogenic disturbances [15,16,17]. Quantitative methods are required to adequately monitor the timing (initiation and termination dates) and duration of migration in waterfowl at the individual, population, and species levels
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