Hydrographic Correlates of Winter Dunlin Abundance and Distribution in a Temperate Estuary

Abstract

From 1989 to 1999,1 examined intra-seasonal and annual changes in the abundance and local dis­ tribution of two groups ofwintering Dunlin (Calidris alpina) occupying different positions along hydrographic gra­ dients in the Tomales Bay estuary, California. Reciprocal translocation of color-banded Dunlin indicated a discrete separation of wintering Dunlin between the north and south of Tomales Bay. Local abundance in both groups was significantly associated with cumulative seasonal rainfall within and among years. Increased variation in annual and intra-seasonal abundance was related to reduced tidal circulation, greater freshwater stream flow, and increased sa­ linity variation. On average, winter Dunlin use peaked later but declined earlier in the southern part of the bay (near the head of the bay) than in the northern part of the bay (near the mouth of the bay), Shifts in distribution associated with changes in hydrographic conditions and weather revealed consistent intra-seasonal changes in hab­ itat preference on temporal scales other than tidal cycles. In the northern part of the bay, changes in habitat pref­ erence by Dunlin corresponded to thresholds of 20-25 cm cumulative rainfall and 0.5-1.0 m" sec" stream flow, whereas in the south bay changes were relatively continuous. Rainfall, wind velocity and direction, and temperature also influenced habitat preferences. Flocking beha\~or dominated over habitat choice at low levels of Dunlin abun­ dance, resulting in contrasting patterns of habitat use relative to overall trends. These patterns suggested the loss of habitat quality as the criterion for patch use. In general, this study indicates that winter shorebird use of temper­ ate estuaries can be predicted by differences in weather and hydrographic regimes, including rates and variances of freshwater inflow, estuarine circulation, and water residence times unique to each system. Received 4 February