Birds are highly mobile; although they are relatively conspicuous and easily identified, their populations are often difficult to estimate effectively. They are, none the less, the most intensively studied species group, and a large amount of data are available on the distribution, ecology and estimated population sizes of most species. A substantial network of experienced volunteers is involved in countrywide bird monitoring programmes such as the Breeding Bird Survey and the Wetland Birds Survey, organised by the British Trust for Ornithology (BTO) in the UK (the latter in association with the Wildfowl & Wetlands Trust, WWT). In some instances it may be possible to incorporate the information provided by these programmes into a site-based monitoring scheme.

The objectives of bird population assessment need to be clearly identified at the outset. Migratory birds may be winter or summer residents, or may only appear on passage between wintering and breeding grounds. Other species are resident all year round but may show seasonal variation in numbers owing to an influx of birds from other areas during the summer or winter. Autumn populations will also include birds that have fledged in that year (many of which will not survive over winter). It is important, therefore, to be able to separate natural population cycles from underlying trends in population size. Population estimates from similar times in each year should be compared, and it may be necessary to calculate 5 or 10 year means to remove ‘noise’ from the data.