My colleague John Underhill-Day, who has died of heart failure aged 74, was an outstanding all-round naturalist who negotiated the purchase for the Royal Society for the Protection of Birds (RSPB) of such flagship reserves as Minsmere in Suffolk and Leighton Moss in Lancashire.

During 17 years (1971-88) as deputy chief reserves officer and head land agent at RSPB head office, he was instrumental in the huge expansion of its reserves, persuading landowners to sell. He was an excellent negotiator, tough but fair. He pioneered a holistic approach to RSPB’s reserve management, for nature in general.

His love was to work in the field and so in 1988 he became RSPB warden at Haweswater in the Lake District, the only place where golden eagles were breeding south of the border. While there he completed a PhD on the breeding status of marsh and Montagu’s harriers in Britain. In 1996 he became site manager for the Arne reserve in Dorset.

The RSPB had to face development threats to the lowland heaths, in which John was proficient, and so it cajoled him into serving as an expert witness at public inquiries: on the Thames Basin heaths, Talbot Heath in Dorset and Lydd airport in Kent. The lawyers were impressed by his magisterial, calm authority. In 2010 John received a 40-year service award from the RSPB.

He was born in St Albans, Hertfordshire, the son of Jack, who ran a family exporting business, and his wife, Muriel (nee Clark). Independent-minded and stubborn, he hated his schooling at Lancing college in West Sussex. He left at 16 and trained to become a surveyor: the top student in the country for his year. The office routine of a surveyor did not suit him and after five years he became warden of Coombes Valley, the RSPB reserve in Staffordshire.

In 2005 John, with his RSPB colleague Durwyn Liley, established Footprint Ecology at Wareham in Dorset, a consultancy specialising in conservation management and people. His small team shared his love of nature.

In later life he was a naturalist on Mediterranean cruises, often joking: “If you say a Latin name with confidence, they believe you.”

Armed with binoculars and butterfly net, he inspired his three children, Tom, Penny and Nick, to love the natural world.

He is survived by his second wife, Jackie (nee Moore), whom he married in 1991, and her children Alistair, Stephen and Dan; by his children, from his first marriage, to Beryl Millbank, which ended in divorce; and by his sister, Janet.