Counting Rooks nests

One of the objectives of the Atlas is to get a good estimate of the number of nests of colonial species, and for us this primarily means Rooks. I have been looking at the total of Rooks nests in each of the tetrads that we will be covering this summer, and am about to send out a list of sites in their tetrad to each observer. The data comes from 2005, and on average there is a turn-over of about 15 rookeries a year, so it may well be that somes of the sites will have been deserted, and that others will have been created, and certainly the number of nests will have changed.
Rooks can only be counted before leafing, which means that those observers ought to do an early April count. This can be done separately from the normal tetrad count, and sent in as a Roving record, or as part of a tetrad count, stopping the clock while the count is made. Counts can often be done with reasonable accuracy from a distance. Some tetrads have only a single rookery, some have up to eight.
To give an idea of the scale, in 2005 we counted 6000 nests, which was 2000 fewer than in 2000, and this did not include any Mendip rookeries.
Casual records of Rookeries will also be of value, simply as a check, and should be sent in as Roving records. The total nests in a rookery inceases over time, as young birds join on the periphery-older birds take the best positions and have laid by the beginning of April, so that counts made in late March will give a very good idea of totals.