Status and distribution of Rook Corvus frugilegus in Wales in 2022/23

The Rook Corvus frugilegus is a well‐recognised colonial breeding species associated with farmed landscapes. Over the last century, Rook specific surveys identified fluctuating populations. The most recent UK survey was in 1996. In the latest BoCC review, the species’ conservation status in Wales was upgraded from green to red. In 2022 and 2023 the Welsh Ornithological Society (WOS) coordinated a countrywide colony survey. Coverage was based on tetrad searches and colony counts, following a stratified random design. To estimate population and distribution size and change, a Random Forest modelling approach was implemented for both 1996 and 2022/23 datasets, using a two‐step approach modelling presence/absence of Rook colonies in each tetrad, followed by the abundance of Rook nests (colony size) where present. These model predictions were applied to all tetrads in Wales and a bootstrapping approach taken to estimate total Rook nest abundance at each spatial scale. Differences in distribution, abundance and change in the average number of breeding pairs per rookery were tested for significance using generalized linear models (GLMs). The current Welsh Rook breeding population was estimated to be 44,127 pairs (95% confidence limits (CI): 41,144‐46,940). This was estimated to represent a decline in the abundance of Welsh Rook breeding pairs nationally and a 5.6% decline in the number of occupied tetrads in Wales since 1996. The average number of breeding pairs per rookery (size of rookeries) has also declined between 1996 and 2022/23, from 25 nests in 1996 (CI: 22–28) to 20 nests in 2022/23 (CI: 19–21), a decline of 20%. There was substantial regional variation in both distribution and abundance change. The drivers of Rook declines in Wales are unclear and will require further investigation if the deteriorating conservation status of Rook in Wales is to be addressed.