Constant Effort Site Report 2025

The FBO Ringing Team run a CES site at Thornwick. CES (Constant Effort Site) is a BTO (British Trust for Ornithology) led project aimed at monitoring bird populations through time, in order to aid effective conservation. It seeks to monitor numbers of birds (abundance), the number of births (breeding success or productivity) and deaths, usually recorded as the number that don’t die (survival).

This data enables the BTO to calculate expected changes in numbers, and at what stage in their lifecycle birds may be affected by environmental changes. They can then try to find out causes for any decline in numbers.

The CES scheme uses comparisons of the numbers of birds caught each year to provide indices of population change, looking at twenty four species in particular. This is done by standardised mist-netting through the breeding season, consisting of twelve visits between May and August.

This year the team completed ten out of the twelve sessions, the table below shows the highlights of the last five years participating in this important project, the species in red are ones highlighted by the BTO as being of specific importance to the scheme.