CHESHIRE AND WIRRAL ROOK SURVEY 2009: UPDATE

The aim of the Survey was to provide an estimate of the current population of Rooks in Cheshire and Wirral. This is because there is concern that the population may have fallen since the first Cheshire & Wirral Atlas in 1978-84.

In March and April 2009, surveyors located rookeries and counted the apparently occupied nests in each rookery. An initial analysis of the data has now been completed. An interesting picture that no-one had anticipated at the beginning of the Survey has emerged and a preliminary account of the Survey findings can be found by following this link.

We already have some useful results from the Survey analysis undertaken so far and it seems that there is a good deal more to learn from the available data, for example looking at trends across the county and comparing the results with the findings of the BTO 1996 national survey. Data review and analysis is continuing. A full account of the findings of the 2009 Rook Survey will appear in the 2009 Cheshire and Wirral Bird Report in due course.

2010 Repeat Rook Survey

In the light of the relatively severe winter of 2009-10, a follow-up nest count was undertaken in spring 2010 to determine if the adverse winter weather may have had a noticeable impact on the county Rook population. A complete recount was not attempted since a reliable indication of any significant population change is achievable through a sample. A request was made to those that took part in the 2009 count to recount those rookeries that were most convenient for them.

In total, rookeries that accounted for 60% of the apparently occupied nests in the 2009 survey were recounted in 2010. A minor population decrease of just -0.15% was determined in 2010 for this sample, indicating no significant adverse impact from the harsh winter conditions. Previous studies have indicated that more significant year-to-year declines in Rook populations may arise primarily from higher than average summer mortality that results when unusually dry conditions adversely affect the accessibility of soil invertebrates that form a significant component of the diet of Rooks. The summer of 2009 was not particularly dry and, if there was a higher than average mortality in the winter 2009-10, this has evidently been offset by other factors such as a lower than average 2009 summer mortality rate.

The 2010 recount provides further evidence of the abandonment of colonies and the establishment of new ones. Of the 201 rookeries identified in the 2009 survey that were revisited in 2010, 7 were found to be abandoned. These losses were compensated by a similar number of new rookery sites identified in 2010.

The repeat survey also showed relatively large fluctuations in the size of a significant proportion of individual Rookeries. Counts declined by 20% or more at around a quarter of Rookeries whilst a 20% increase or more was found at a similar proportion of sites. Large fluctuations in Rook colony size has been observed in previous studies. It seems that this results from different rates of new recruitment into different colonies. Whilst adult birds are generally faithful to their colony and return to the same Rookery each year, the same level of colony fidelity is not observed in young Rooks. Birds mix together in larger communal roosts during the winter and, when breeding for the first time, young Rooks will not necessarily join their natal colony. With some colonies gaining more than their fair share of new recruits and others receiving less, 20% fluctuations in colony sizes are not surprising, in particular given the typical adult survival rate of around 80%.

Mark Eddowes
October 2010