Acknowledgements and funding

We thank Nature Seychelles for facilitating fieldwork on Cousin Island. This study would not have been possible without the contribution of many fieldworkers, lab technicians, students and database managers during the whole study period. T.J.B. was funded by the UKRI Biotechnology and Biological Sciences Research Council Norwich Research Park Biosciences Doctoral Training Partnership (BB/M011216/1). The long-term Seychelles warbler study was funded by the Natural Environment Research Council (NERC) grants to D.S.R (NE/F02083X/1) and D.S.R and H.L.D (NE/K005502/1), and NWO grants from the Dutch Science Council (NWO) to J.K., with D.S.R. and H.L.D. (NWO-ALW 823.01.014, NER/I/S/2002/00712, NE/I021748/1, 854.11.003 and 823.01.014).