Connectivity between flyway populations of waterbirds: assessment of rates of exchange, their causes and consequences

Abstract

Summary Conservation and management of migratory waterbirds use flyway populations as the basic unit, and knowledge of the delineation, rate of exchange and gene flow between populations is fundamental. However, for the majority of global flyway populations, information is too fragmentary to address connectivity between populations and, hence, insufficient to inform management. We investigated the demographic connectivity between the eastern (breeding in Svalbard and wintering in Denmark, the Netherlands and Belgium) and western (breeding in Greenland or Iceland and wintering in Britain) flyway populations of pink‐footed geese Anser brachyrhynchus based on resightings of marked geese from both populations. Previous genetic analyses suggested a modest gene flow between the two populations. Capture–recapture analysis conservatively estimated that mean annual movement probabilities were low (eastern to western population: 0·071%, 95% CI = 0·033–0·15%; western to eastern: 0·076%, 95% CI = 0·031–0·18%). Movement probability from eastern to western flyway populations increased in years with high snow cover in the southernmost winter range in Belgium. Life histories of exchanged individuals from eastern to western (32 different individuals during 1988–2010) revealed that the majority entered Britain via Belgium and the Netherlands during winter; some returned to the eastern population via Belgium and/or the Netherlands, others moved northwards in Britain during the spring and appear to have migrated directly from Britain (western population) to Norway (eastern population). None of the birds from the eastern population emigrated permanently, but some individuals turned up in Britain in consecutive years. Out of nine individuals switching from western to eastern flyway populations, three returned to Britain; the others were not subsequently resighted. An alternative winter strategy and spring flyway over Britain to Norway is suggested, used by hundreds to thousands of eastern birds, particularly following severe winters. Thus, the two populations currently appear to be demographically closed; low genetic connectivity probably reflects dispersal over longer time. Synthesis and applications. Current initiatives to internationally manage the eastern population of pink‐footed geese do not need to consider net immigration in predictive harvest models. For waterbirds in general, a targeted approach to evaluate connectivity, using classic marking studies in combination with molecular methods and focussed sampling on breeding grounds, is recommended to better underpin management decisions at population levels.