This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/IBI.12801 This article is protected by copyright. All rights reserved Climatic conditions during migration affect population size and arrival dates in an Afro-Palearctic migrant Running title: Climate and demography in Sand Martins THOMAS O. MONDAIN-MONVAL, 1 * KEVIN BRIGGS, 2 JOHN WILSON 3 & STUART P. SHARP 1 1 Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, UK 2 1 Washington Drive, Warton, Carnforth, LA5 9RA Lancashire, UK 3 40 Church Hill Avenue, Warton, Carnforth, LA5 9NU Lancashire, UK *Corresponding Author: t.mondain-monval@lancaster.ac.uk We would like to thank all members of the Lancaster and District Birdwatching Society who contributed sightings and Peter Cook for providing their data. Rob Robinson and Ian Hartley provided useful feedback on an earlier version of the manuscript, and Rob also advised on the statistical analyses. Thanks to the Editor, Associate Editor and anonymous reviewers whose comments helped to improve the manuscript. Accepted Article