LARGE numbers of shorebirds that breed in North America spend the winter in the region of the Bay of Paracas, 150 miles south of Lima, Peru. During a short period in October and November 1965 I had the opportunity to observe some aspects of the feeding behavior of mixed flocks of two very similar species, the Semipalmated Sandpiper (Ereunetes pusillus) and the Western Sandpiper (E. mauri), which has a considerably longer bill. They were feeding in a small harbor-like inlet with a narrow entrance, situated at La Puntilla, Pisco, about 2 miles north of the Bay of Paracas. The tides caused important changes in this habitat, and these varied considerably from day to day; the lowest tides virtually emptied the inlet, while the highest of the low tides left the inlet bottom at least one third covered with water. At low tide, three zones could be distinguished; water, wet mud (shiny), and dry mud (dull). These zones are presumably equivalent respectively to zones D and E, zone B, and zone A in Figure 3 in Recher's study of the feeding of shorebirds (Ecology, 47: 393-407, 1966). The extent of the three zones changed continually as the water flowed in and out of the inlet, and as wet mud dried on the outgoing tide and dry mud became wet mud on the incoming tide. The numbers of birds in the inlet varied from day to day; in late October and the first half of November there were usually between 20 and 100 Semipalmated Sandpipers (average 33) and between 3 and 40 Western Sandpipers (average 8). The Western Sandpipers were frequently in two or three loose groups within the flock of Semipalmated Sandpipers, and the two species fed over the same general area, frequently only a meter or so apart. I noticed only occasional interor intraspecific aggression. My objective was to determine the extent to which these two species differ in their exploitation of the feeding habitat. As close as possible to the time of low tide, I made a series of counts of the flock in quick succession, noting first how many of each species were in water, then how many were on wet mud, and finally how many were on dry mud. Birds at the boundaries of the defined substrates were ignored, for instance birds standing on wet mud but feeding in water. These counts were repeated several times on each of 6 days. The totals for each day (Table 1) show that on most days the highest percentage of both species fed on wet mud. On all days the Western Sandpiper tended to be relatively more abundant in water, and the Semipalmated Sandpiper to be more abundant on dry mud. The consistency of this trend indicates a real difference in zone
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Jan 1, 1971
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