This study focused on understanding the variations in the visitation characteristics of shorebirds (Calidris spp.) between 1990 and 2000 at Nakdong Estuary, Korea. The dataset was collected within 2 long-term monitoring programs: (1) during early 1990 (May 1989 to April 1993) and (2) mid-2000 (May 2002 to April 2008).
During these 2 periods, 86,667 birds representing 9 species of shorebirds were recorded at the Nakdong Estuary. Specifically, 22,124 red-necked stints (Calidris ruficollis), 6 long-toed stints (C. subminuta), 4 pectoral sandpipers (C. melanotos), 24 sharp-tailed sandpipers (C. acuminata), 61,627 dunlins (C. alpina), 4 curlew sandpipers (C. ferruginea), 230 red knots (C. canutus), 1106 great knots (C. tenuirostris), and 1542 sanderlings (C. alba) were observed. Of these 9 species belonged to the Calidris genus; 44,218 individuals (of the total 86,667 observed individuals) were recorded during the first study (early 1990), and included 18,635 red-necked stints, 6 long-toed stints, 4 pectoral sandpipers, 11 sharp-tailed sandpipers, 24,482 dunlins, 42 red knots, 536 great knots, and 502 sanderlings. The remaining 42,449 individuals were observed during the second study (mid 2000), and included 3489 red-necked stints, 13 sharp-tailed sandpipers, 37,145 dunlins, 4 curlew sandpipers, 188 red knots, 570 great knots, and 1040 sanderlings.
A greater number of individuals were observed during early 1990 compared to mid-2000, with a high number of red-necked stints being documented in the first study (early 1990). Since individuals belonging to the genus Calidris primarily prefer mudflat areas, the decline in species numbers is probably related to factors leading to loss of habitat, such as water contamination and land reclamation. These activities occurred because of the development of the Myungji residential areas and the brackish water zones near Daema-deung (DMD). However, there was no significant difference in the number of observed individuals between the 2 study periods.
The average numbers of birds observed in the 5 different regions of the estuary were 2010.3 in Daemadeung, 2952.7 in Jangja · Shinjado, 990.8 in Saja · Doyodeung, 1012.0 in Lower Ulsukdo, and 109.0 in Ulsukdo, with an overall average of 1415.0 birds across these 5 regions. There was a significant difference in the number of birds recorded across the 5 regions. For instance, a greater number of birds was observed in Jangja-Shinjado and Daemadeung compared to those in the other regions.
This study showed that the spatial distribution and species abundance of shorebirds inhabiting Nakdong estuary are strongly influenced by the size of the tidal flats located near the sandbar, in addition to changes in the environmental characteristics of this habitat.