She first visited Bardsey Island - the nature reserve in North Wales which is famous for its breeding sea-birds and the rare red-billed crow, the chough - as a volunteer cook at its recently founded bird observatory in 1957.
From cook she graduated to council member of the observatory and began a study of the island's chough population. In Britain this is a very rare breeding bird found in the Celtic fringes in west Wales, the west of Ireland and south-west Scotland. Later that interest led to the setting up of a national census of the species through the BTO. In 1976 she was the prime mover of the appeal that led to the purchase of the island and the establishment of the Bardsey Island Trust.
She was also active in her native Buckinghamshire. She played a key role in the development of the Berks, Bucks and Oxon Naturalists Trust, and was instrumental in creating a series of nature reserves in the area - choosing the sites, persuading landowners and farmers to co-operate and raising money for purchase. In the 1970s she founded the Bucks Bird Club and served on the councils of both the RSPB and the BTO and was vice-president of the latter. Though she was always keen to promote others, honours did come to her. She was Bernard Tucker Medallist of the BTO in 1968 and in 1995 received the highest honour the conservation world can offer, the Christopher Cadbury Medal of the Royal Society for Nature Conservation.
Born Susan Stewart-Liberty in 1914, she grew up in the Chiltern village The Lee, owned and remodelled by her family, whose business was Liberty's, the Regent Street store. Her childhood was a blend of influences which included the circles of artists and craftspeople associated with the shop, the natural world of the Chilterns, horses and hunting with the Old Berkeley, and a large and close-knit family. An early marriage took her to Northern Ireland where John Cowdy, her husband, worked in his family's linen business. He, like her, was an accomplished point-to-point rider. Susan Cowdy, soon with three small children, did not become an isolated young mother. She had a wide circle of varied friends, and childcare became largely a matter for the nanny while she bought, broke and sold horses to supplement the family income. The family returned to The Lee during the Second World War, and it was after her children left home that her childhood interest in natural history and especially birds came to the fore.
Susan Cowdy had the capacity to get people to enjoy doing exactly what she wanted them to do. She herself was always willing to turn her hand to anything. She had little relevant formal education, but through talking, reading and observation she became as knowledgeable as many professionals.
Her home was the hub of this activity. People came and went continuously. Round the kitchen table might be a near neighbour responsible for the catering at the local bring-and-buy sale; a prominent figure in the international conservation movement; a teenager in need of a temporary refuge; a plant ecologist; a lobster fisherman from Lleyn en route to the Boat Show in London: all engaged with Cowdy in animated discussion of Bardsey Island politics or the likely winner of the Cheltenham Gold Cup.
Unlike most naturalists of her generation she did not travel the world when cheap airfares became available. The British Isles provided more than enough to delight her eye and feed her curiosity. The New Year would usually find her on the north Norfolk coast or at Gibraltar Point in Lincolnshire, eyes watering in the easterly blast as she identified waders. Spring was the time for the flowers of the Burren, the choughs of western Ireland and visits to Irish relatives - Bardsey drew her in the migration season. But perhaps she was happiest of all walking the dogs in the beech woods and on the downland of the Chilterns she came to love in her childhood.
People always interested her as much as the natural world and almost to the end of her life she would delight local audiences with scurrilous talks about the five generations of her family who had lived in her Chiltern village.
Susan Stewart-Liberty, natural historian and conservationist: born The Lee, Buckinghamshire 6 August 1914; MBE 1981; married 1935 John Cowdy (died 1974; one son, two daughters); died Stoke Mandeville, Buckinghamshire 9 July 1996.