Estimating the abundance and hatching success of breeding Curlew Numenius arquata using survey data

Counts of individual Curlew and estimates of breeding pairs from 'field-by-field' surveys were highly correlated with the numbers of nesting pairs, ns determined by intensive studies on 27 sites located on four grassland-dominated study areas. The mean count of individual Curlew over three standard survey visits to each site was used to estimate numbers of nesting pairs. This estimate exceeded the number of nesting pairs on all study areas (as assessed by intensive studies) by 12% but was move accurate than the maximum number of pairs estimated from the three survey visits on each site (the previous convention for estimating breeding pairs). Estimates of the number of pairs hatching chicks were assessed by recording alarm-calling Curlew during late survey visits. The maximum number of alarm-calling pairs was used to estimate the number of pairs hatching chicks, overestimating this by 54%. Three of the study sites were adjacent to extensive moorland which produced overestimates during surveys because moorland nesting birds fed and Zed broods onto these sites. Omitting these sites from consideration reduced the degree of overestimation to 1% for the number of pairs and 7% for the number of pairs hatching chicks.