Migration confers winter survival benefits in a partially migratory songbird

Abstract

To evolve and to be maintained, seasonal migration, despite its risks, has to yield fitness benefits compared with year-round residency. Empirical data supporting this prediction have remained elusive in the bird literature. To test fitness related benefits of migration, we studied a partial migratory population of European blackbirds (Turdus merula) over 7 years. Using a combination of capture-mark-recapture and radio telemetry, we compared survival probabilities between migrants and residents estimated by multi-event survival models, showing that migrant blackbirds had 16% higher probability to survive the winter compared to residents. A subsequent modelling exercise revealed that residents should have 61.25% higher breeding success than migrants, to outweigh the survival costs of residency. Our results support theoretical models that migration should confer survival benefits to evolve, and thus provide empirical evidence to understand the evolution and maintenance of migration.

Population and evolutionary dynamics in spatially structured seasonally varying environments

“…Migratory individuals can make long‐ or medium‐distance geographical migrations (B–E), or medium‐distance altitudinal migrations (F–H), or short‐distance migrations between adjacent but distinct habitat types (I, J). Geographical migrations occur in (B) wandering albatross ( Diomedea exulans ; Weimerskirch et al, ); (C) tiger shark ( Galeocerdo cuvier ; Papastamatiou et al, ); (D) European shag ( Phalacrocorax aristotelis ; Grist et al, , ); (E) Skylark ( Alauda arvensis ; Hegemann et al, ), and numerous other birds [including blackbirds, Turdus merula (Fudickar et al, ; Zúñiga et al, ), and American kestrels, Falco sparverius (Anderson et al, )]. Altitudinal migrations occur in (F) elk ( Cervus elaphus ; Hebblewhite & Merrill, ; Eggeman et al, ) and other ungulates (e.g.…”

Migratory connectivity in the context of differential migration

“…Differential migration is a widespread phenomenon in the animal kingdom and can take many forms, from differences in the timing of migration to differences in migration routes: subgroups may segregate during relatively short periods, during migration [10,[13][14][15], during non-breeding residency [16,17] or for most of the year [18,19] (figure 1). Similarly, the spatial scales of segregation may vary from local segregation such as the use of microhabitats [20], to regional (several tens to a few hundreds of kilometres) when different stopover or wintering sites are used [16,18], to continental, when subgroups use entirely different flyways [21].…”

Fitness consequences of different migratory strategies in partially migratory populations: A multi‐taxa meta‐analysis

“…The potential costs of migration are high: migratory individuals may encounter unfamiliar environments with novel threats, as well as the energetic costs of movement (Wikelski et al, ), predation risks (Lindström, ; Ydenberg, Butler, Lank, Smith, & Ireland, ) and temporal investment to the detriment of time otherwise invested in breeding fitness (Alerstam et al, ). The biological processes underlying the evolution of migration are little known (Griswold, Taylor, & Norris, ; Townsend, Frett, McGarvey, & Taff, ; Vélez‐Espino, McLaughlin, & Robillard, ), but in order to have evolved, migration must—in sufficient instances—offer a benefit relative to not migrating (‘residency’ hereafter) to either breeding success or survival (Griswold et al, ; Lundberg, ; McKinnon et al, ; Zúñiga et al, ).…”

“…Partial migration represents a behavioural dimorphism; in order for it to be maintained, either the two strategies yield equivalent fitness returns—an evolutionary stable state—or they confer overall balanced relative benefits which differ according to circumstance, known as a conditional strategy (Chapman et al, ; Kokko, ; Lundberg, ). It follows, therefore, that in partially migratory populations residency may offer complementary fitness benefits to those offered by migration (Lundberg, ; Zúñiga et al, ). In the case of conditional strategies, these may refer to individual states such as sex or body condition (Hegemann, Marra, & Tieleman, ; Warkentin, James, & Oliphant, ), or external conditions, such as population density (Grayson & Wilbur, ) or environmental conditions (Chapman et al, ; Lack, ; Lundberg, ; Meller et al, ).…”

“…Two of the main demographic parameters controlling population size are breeding success and survival (Griswold, Taylor, & Norris, ; Lundberg, ) , though the extent of the influence of each parameter on population size may differ between populations (Morrison, Robinson, Clark, Risely, & Gill, ). Theories surrounding the maintenance of partial migration have hypothesized that the balance of benefits between migration and residency hinges on differential advantages to survival versus breeding success between the strategies (Griswold et al, ; Lundberg, ; Zúñiga et al, ). These generally predict that migration confers survival benefit as it allows individuals to escape unfavourable climatic conditions and low resource abundance, while residency promotes breeding success through early access to better resources—such as territories or breeding locations (Chapman et al, ; Kokko, ; Lundberg, ).…”