Demographic outcomes of diverse migration strategies assessed in a metapopulation of tundra swans.

Craig R Ely,Brandt W Meixell

doi:10.1186/s40462-016-0075-8

Abstract

BackgroundMigration is a prominent aspect of the life history of many avian species, but the demographic consequences of variable migration strategies have only infrequently been investigated, and rarely when using modern technological and analytical methods for assessing survival, movement patterns, and long-term productivity in the context of life history theory. We monitored the fates of 50 satellite-implanted tundra swans (Cygnus columbianus) over 4 years from five disparate breeding areas in Alaska, and used known-fate analyses to estimate monthly survival probability relative to migration distance, breeding area, migratory flyway, breeding status, and age. We specifically tested whether migratory birds face a trade-off, whereby long-distance migrants realize higher survival rates at the cost of lower productivity because of reduced time on breeding areas relative to birds that migrate shorter distances and spend more time on breeding areas.ResultsAnnual migration distances varied significantly among breeding areas (1020 to 12720 km), and were strongly negatively correlated with time spent on breeding areas (r = −0.986). Estimates of annual survival probability varied by wintering area (Pacific coast, Alaska Peninsula, and Eastern seaboard) and ranged from 0.79 (95%CI: 0.70–0.88) to 1.0, depending on criteria used to discern mortalities from radio failures. We did not find evidence for a linear relationship between migration distance and survival as swans from the breeding areas with the shortest and longest migration distances had the highest survival probabilities. Survival was lower in the first year post-marking than in subsequent years, but there was not support for seasonal differences in survival. Productivity varied among breeding populations and was generally inversely correlated to survival, but not migration distance or time spent on breeding areas.ConclusionsTundra swans conformed to a major tenet of life history theory, as populations with the highest survival generally had the lowest productivity. The lack of a uniform relationship between time spent on breeding areas and productivity, or time spent on wintering areas and survival, indicates that factors other than temporal investment dictate demographic outcomes in this species. The tremendous diversity of migration strategies we identify in Alaskan tundra swans, without clear impacts on survival, underscores the ability of this species to adapt to different environments and climatic regimes.

Highlights

Migration is a prominent aspect of the life history of many avian species, but the demographic consequences of variable migration strategies have only infrequently been investigated, and rarely when using modern technological and analytical methods for assessing survival, movement patterns, and long-term productivity in the context of life history theory
Intra-population variance in migration distance was high for LAP swans, as most birds did not migrate away from Alaska, and for BBL swans which used two different terminal wintering areas that were at different distances from the breeding area (Figs. 1 and 2)
Time allocation and life history trade-offs Our top survival model did not include migration distance as a parameter, and since migration distance was equitable with time spent on breeding areas (r2 = 0.97; Table 1); the latter, and its reciprocal were not predictive of survival probability

Summary

Introduction

Migration is a prominent aspect of the life history of many avian species, but the demographic consequences of variable migration strategies have only infrequently been investigated, and rarely when using modern technological and analytical methods for assessing survival, movement patterns, and long-term productivity in the context of life history theory. Understanding the demographic consequences of migration has long been a goal of avian ecologists, as identifying the fitness costs associated with different behaviours should further our understanding of mechanisms driving the evolution and maintenance of avian life history strategies [5, 6]. The energy cost of long migrations is undeniably high with potentially large negative impacts on reproduction, as migrants must balance between using energy for flight and storing reserves for reproduction [9, 10]. Such costs are likely greatest in pure capital breeders, that unlike income breeders, cannot offset the energetic burden of migration by foraging on breeding areas. The cost of migration, especially long-distance migration, on survival is less clear but of great interest, because the population dynamics of long-lived species are generally predicated on adult survival [11, 12]

Methods

Results

Discussion

Conclusion