Mark Rehfisch was one of the first ecologists to realize that the impacts of climate change on wintering waders and wildfowl in the UK would tend to move birds eastwards rather than northwards. He recognized that the isotherms in winter lie north–south as opposed to east–west, as occurs in the summer. A series of papers (Rehfisch et al. 2004a, Austin & Rehfisch 2005, Maclean et al. 2008) confirmed these ideas, showing that the wintering populations of birds such as Eurasian Oystercatcher Haematopus ostralegus, Common Ringed Plover Charadrius hiaticula and Dunlin Calidris alpina were declining on western estuaries while increasing in the east. This allowed the birds to take advantage of better feeding grounds in the east, while not having to face the risks of prolonged freezing conditions. Non-estuarine shorebirds also shifted distributions northwards and eastwards, which was related to being closer to their breeding grounds. These papers were just some of the many that Mark wrote or contributed to, based on the extensive citizen-science datasets held by the British Trust for Ornithology (BTO) and which he helped to create. For 14 years, Mark led the BTO’s Wetlands and Coastal Team and drove the push towards making the schemes such as the Wetlands Birds Survey and Low-tide Count Scheme much more effective. He did this by moving the systems onto large databases, allowing more efficient collation and verification, so that they could be analysed increasingly quickly over the years. He also instituted the use of Geographic Information Systems and encouraged the use of stratified randomized waterbird surveys. This was greatly appreciated by the partners of the schemes, including the UK Government, through the Joint Nature Conservation Committee (JNCC), the Wildfowl and Wetlands Trust (WWT) and RSPB. He also saw the importance of increasing the coverage of wetland bird monitoring to non-estuarine shorebirds and gulls, as well as non-native species – leading on the development of surveys of these groups and dealing with some formidable operational and practical problems with great success. Mark was born in Omdurman in Sudan, where his father was an anthropologist working out of Khartoum and he was proud of his French heritage from his mother. He spent his teenage years in France and Hull (where his father was a professor) and was introduced to birdwatching and ringing by Dr Stuart Walker, the local GP, at Hornsea Mere, joining the BTO as a member at the age of 13. He studied Zoology at Hull, gaining his BSc in 1983, and was winner of the Professor R.K. Murton Prize in Zoology for his thesis on invertebrate prey of mudflat birds. He then went to Lake Ichkeul, Tunisia, as a research assistant from University College London (UCL), where he undertook research on Greylag Geese Anser anser, monitored up to a quarter of a million wintering waterbirds, collected hydrological and botanical data, and co-authored a number of papers derived from his work there. His PhD (1990), also at the University of Hull, combined his interest in invertebrates and birds by exploring the chironomid populations of man-made lagoons of Blacktoft Sands RSPB reserve and how they might be optimized for feeding ducks and waders. He came up with what he called a ‘reverse sombrero’ design – shallow areas for feeding, but a deeper central area, where the chironomids could retreat under adverse freezing conditions in winter or dry summer conditions (Rehfisch 1994). After a brief stint as a teacher in Jeddah in Saudi Arabia, which arose when an opportunity to study Houbara Bustard Chlamydotis undulata in Taif fell through, he applied for an Estuarine Biologist post at the BTO in 1990 – his success might partly have been due to the finishing sentence on his application letter: ‘Finally, my main qualification is a love of being cold and muddy’! His first job involved radio-tracking Pintail Anas acuta on the Mersey estuary – working round-the-clock with his colleague Innes Sim, sleeping in cars and following the Pintail and Teal Anas crecca from dawn to dusk. When he returned to the BTO, he worked with Nigel Clark in the Habitats Advisory Unit, undertaking or supervising a wide range of projects, often on potential barrage schemes and wind turbine developments. In 1995 he became BTO’s Head of the Wetlands and Coastal Ecology Unit where he began, among other things, to work on the potential impacts of climate change. One of the most remarkable projects was working with the Centre for Ecology and Hydrology on developing models of how estuary shapes and sediments might change with projected sea-level rise. In addition to producing papers on population estimates for a range of different bird groups, other important studies included an analysis of ringing recoveries to show the optimum spacing for refuges for wintering waders (Rehfisch et al. 1996); how a sudden loss of wintering habitat affects Common Redshank Tringa totanus survival (Burton et al. 2006); the impacts of bird populations on the microbiological quality of bathing waters (Wither et al. 2005); and a number of more general papers on the impacts of climate change on wintering waterbirds (e.g. Austin & Rehfisch 2003, Robinson et al. 2009). In total, he produced over 50 peer-reviewed scientific papers, as well as many research reports for the BTO. His remarkable work ethic, combined with a highly supportive attitude to his staff, led to a substantial increase in the scientific output and reputation of the BTO team under his leadership. While at the BTO, he spent eight years as a member and then Chair of the BOU’s Meetings Committee, the latter role as a Trustee on BOU Council. He successfully developed a wide ranging programme of important and well-attended conferences. The Proceedings of one of which, on Climate Change and Coastal Birds, he edited as a supplement to Ibis (Rehfisch et al. 2004b). He remained a dedicated BOU member and regular attendee at BOU meetings and conferences. He also organized a number of symposia at meetings of the International Ornithological Congress (IOC) and of the European Ornithologists’ Union (EOU). In 2009, he took on the role of Director of Development at the BTO, and oversaw the wide range of contractual work being undertaken by the BTO, helping to diversify the Trust’s income streams and ensure that it was on a sound footing In 2012 Mark decided to look for a new challenge, which was to work in the commercial sector and take up the position as Head of Ornithology and Associate Director at the ecological consultancy APEM. The focus of his scientific work for APEM was on the application of aerial surveys to the assessment of the potential impacts of offshore wind farms on marine birds. During his time with APEM the development of digital imaging sensors, carried as multiple camera arrays on fixed-wing aircraft, led to the ability to cover much larger areas of sea at higher resolution and lower cost. Mark was concerned to ensure that alongside these greater operational efficiencies, there were improvements in the measurement of bird flight heights and of the power to detect change in bird populations before and after an offshore wind farm was constructed. He also led APEM’s own investigations into the use of airborne LiDAR to measure bird flight heights. His long-term involvement in understanding non-breeding wader populations was not neglected while at APEM and he was the instigator of a trial of digital aerial imagery for measuring wintering wader populations at low tide on The Wash, taking part in the ground-truthing himself in February 2018. In addition to being a deeply committed professional ornithologist, he loved birdwatching and, although not a twitcher, he did seem to know his list to a surprising degree of accuracy! He was also passionate about conservation more widely, supporting the World Land Trust and for the last decade or so, never flying – only travelling by boat or train. He would also wag his fingers at drivers of large SUVs or 4WD gas guzzling vehicles which, surprisingly, never got him into trouble! He was a lover of fine wine, fine dining, rugby, Leeds United FC, jazz and sub-aqua. He was a great conversationalist with a wicked sense of humour, because, on any topic he would hold interesting and challenging views that would lead to lively discussions – and often coming round to birds or the natural world. He showed huge courage in dealing with his cancer, working passionately throughout and enjoying birding trips out to the East Anglian coast whenever possible. He was immensely proud of his wife, Kay, and their daughters Anna and Natasha, and he will be greatly missed by all his many friends and colleagues.