Why is timing of bird migration advancing when individuals are not?
Recent advances in spring arrival dates have been reported in many migratory species but the mechanism driving these advances is unknown. As population declines are most widely reported in species that are not advancing migration, there is an urgent need to identify the mechanisms facilitating and constraining these advances. Individual plasticity in timing of migration in response to changing climatic conditions is commonly proposed to drive these advances but plasticity in individual migratory timings is rarely observed. For a shorebird population that has significantly advanced migration in recent decades, we show that individual arrival dates are highly consistent between years, but that the arrival dates of new recruits to the population are significantly earlier now than in previous years. Several mechanisms could drive advances in recruit arrival, none of which require individual plasticity or rapid evolution of migration timings. In particular, advances in nest-laying dates could result in advanced recruit arrival, if benefits of early hatching facilitate early subsequent spring migration. This mechanism could also explain why arrival dates of short-distance migrants, which generally return to breeding sites earlier and have greater scope for advance laying, are advancing more rapidly than long-distance migrants.
School of Biological Sciences, University of East Anglia, , Norwich Research Park, Norwich NR4 7TJ, UK, Conservation Science Group, Department of Zoology, University of Cambridge, , Downing Street, Cambridge CB2 3EJ, UK, British Trust for Ornithology, , The Nunnery, Thetford, Norfolk IP24 2PU, UK, Farlington Ringing Group, Solent Court Cottage, Chilling Lane, Warsash, Southampton SO31 9HF, UK, South Iceland Research Centre, University of Iceland, , Bankavegi, IS-800 Selfoss and Gunnarsholt, IS-851, Hella, Iceland.
Jennifer A
JA
Gill
José A
JA
Alves
William J
WJ
Sutherland
Graham F
GF
Appleton
Peter M
PM
Potts
Tómas G
TG
Gunnarsson
D000222:Adaptation, Physiological; D025041:Animal Migration; D000818:Animals; D001717:Birds / Q000502:physiology*; D057231:Climate Change; D005843:Geography; D009425:Nesting Behavior; D011157:Population Dynamics; D012621:Seasons; D013997:Time Factors
migratory behaviour; phenology; population change; shorebird; timing of breeding