Common bird monitoring schemes have become an important tool in conservation biology because they provide useful information for assessing spatial and temporal variations of bird populations. However, recorded data may suffer from several observational procedures that cause error. In recent years, a robust mathematical framework has been developed to control for potential confounding factors affecting the assessment of the actual spatial and temporal variability of bird populations. Surprisingly few attempts have been made to check the effectiveness of current methodology empirically and thus to determine to what extent monitoring scheme data can provide accurate estimates of actual bird abundances. To check the effect of intra-annual variability of bird counts, I conducted daily surveys of House Martin Delichon urbicum and Common Swift Apus apus populations along a 2-km transect line in Tres Cantos (Madrid, Spain) between March and September 2005. The data recorded perfectly mimicked the information collected every spring by observers participating in the common bird monitoring schemes of Spain. Computer simulations with my data showed that the probability of recording the actual annual maximum of individual numbers was less than 9% by the available protocols. Simulations of protocols with increased survey frequency demonstrated that three or four surveys per year would be enough to increase the accuracy of recorded House Martin abundance significantly. However, at least weekly surveys in spring would be required to produce acceptable annual estimates of the abundance of the Common Swift. My results suggest that two surveys per spring are not enough in the studied species, due to the huge between-day variability of records. Such variability is primarily caused by the nature of the count data and the seasonal patterns of occurrence/ detectability of individuals. Abundance data for swifts and house martins recorded in common bird monitoring schemes should thus be used cautiously due to its potential inaccurary. —Gordo, O. (2018). Are two days enough? Checking the accuracy of the survey protocols used in common bird monitoring schemes. Ardeola, 65: 41–52.
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Vol. 65 • No. 1