While numerous censuses are conducted in Rook (Corvus frugilegus) populations, there is still a lack of comprehensive evaluations regarding the methodologies employed and factors influencing population estimates. Here, we assess patterns of nest number variation in colonies through the breeding season and examine some factors influencing this variability. We conducted four censuses in 18 colonies of the entire Spanish breeding population of Rook spread out over the breeding season, from 18th March to 11th April. The total population's nests count increased by 22% over the census period, with the highest count of 1,877 nests recorded on the last census date. Number nests within the rookeries exhibited fluctuations throughout the census period, with an average variation of 12%, predominantly decreasing over time; variations were higher during the second half of March compared to the first half of April. The temporal variability in rookery nest numbers was found to be independent of colony size, rookery type or census technique. However, it was positively influenced by the number of nests in nearby rookeries located within a 6 km radius, likely due to immigration and emigration processes. While most rookeries (72%) reached their peak nest numbers on the last census day (April 11th), the timing of this peak varied among colonies. This asynchrony leads to variations in the total population nest counts, depending on whether we base it on a single census date or select different dates when colonies reach their maximum nest numbers. Our results emphasize the importance of considering intra-colony temporal variations during the breeding period in census surveys to ensure robust comparisons among colonies and of the total population over time.