Census methods for breeding black-necked grebes Podiceps nigricollis Metoder til optaelling af ynglende sorthalset lappedykker Podiceps nigricollis
Dansk Ornitologisk Forenings Tidsskrift 932: 82-84
In this study, results of counts of adult Black-necked Grebes from the shore and from a rowing-boat were compared with the number of nests. The study was made in the vegetation-rich Lake Brabrand in Jutland. Repeated nest-counts suggested 32 pairs in 1997 and 39 pairs in 1998. Counts from nine vantage points along the shore yielded only 35-50% of the breeding numbers (Fig. 1), simply because many grebs were hidden by dense reeds and rushes. Counts from a boat in the first June decade, however, when most grebes were incubating, estimated breeding numbers with an accuracy of 1-3%. Boat counts in late June only yielded 60-70% of the breeding numbers because failed breeders were leaving the area. In conclusion, a reliable estimate of the breeding numbers could be obtained by counting the numbers of adult Black-necked Grebes from a rowingboat during the incubation period. Counts from a boat were repetable from year to year, reduced the time effort to one half of what was needed for nest-counts, and caused less disturbance.