Bird species data from the UK Environmental Change Network (ECN) lowland terrestrial sites. These data were collected, using the British Trust for Ornithology (BTO)'s Common Bird Census methodology (CBC), at ECN's lowland terrestrial sites using a standard protocol. This protocol was abandoned in favour of the Breeding Bird Survey (Rennie et al (2017) UK Environmental Change Network (ECN) bird data: 1995-2015 https://doi.org/10.5285/5886c3ba-1fa5-49c0-8da8-40e69a10d2b5 ) in 1999. However, some sites continued to follow this protocol for a number of years after 1999 to allow comparison with the Breeding Bird Survey data. The CBC uses a mapping method in which a series of visits are made to all parts of a defined plot during the breeding season and contacts with birds by sight or sound are recorded on large-scale maps. Information from the series of visits is combined to estimate the number of territories found. Annual data are recorded but the date ranges available are variable for each site. ECN is the UK's long-term environmental monitoring programme. It is a multi-agency programme sponsored by a consortium of fourteen government departments and agencies. These organisations contribute to the programme through funding either site monitoring and/or network co-ordination activities. These organisations are: Agri-Food and Biosciences Institute, Biotechnology and Biological Sciences Research Council, Cyfoeth Naturiol Cymru - Natural Resources Wales, Defence Science & Technology Laboratory, Department for Environment, Food and Rural Affairs, Environment Agency, Forestry Commission, Llywodraeth Cymru - Welsh Government, Natural England, Natural Environment Research Council, Northern Ireland Environment Agency, Scottish Environment Protection Agency, Scottish Government and Scottish Natural Heritage.