JPMyers 1974 Calidris fuscicollis Mouth of Canal 4 into Laguna Mar Chiquita, 21 Km Nof Mar Chiquita by road, Pto de Mar Chiquita, Pcia de Buenos Aires, Argentina 21 March (contd) used for any examination of geographic variation, as the population is probably composed of birds which spent time considerably further south. What an incredible sight. Arroyo Chico, Estancia Medland, 35 Km S. of Juncalao by road, Pto de Urdanaga, Pcia de Bs Aires, Argentina 22 March 12 WR total on 1645-1716 linear grid sample. RyLoo was on territory. Witnessed one timberd interation on subunit B, in 2 west across from B (border). WR along the grid are spaced out as if they are all territorial. Counted WR from Rk 11 (bridge) to fence between Potero's 19 chico + 23 (i.e. N end of linear grid). 94 total, including one very tight flock of 21 and another of 5, both in 19 chico. All other birds in 19 chico were exquisitely spaced. As within 19 chico I found 67 total WR, 24/64 or 38% circle flocking, leaving the remaining 64% as probable territorial. Notable. On the linear grid, between subunits 10 and 4, Rolo, RyLoy, RyLpr, and RyLoo were on their territory. The span between B and 4, which (as I remarked before) was at one time the haunt of a sizable flock, is now fairly evenly divided by territorial birds. "Running down" counts suggest 6-8 WR spaced along the 200 m stretch, probably not all territorial 23 March only 8 WR in morning sample of linear grid (0830). Continued W to area of RyLgr, 500 m N of the fence separating Potero 19 grands from 23. I watched RyLgr for 30 minutes, finding it to be twittering whereas before (3/15) it appeared to be foraging within a flock. Unfortunately, earlier, although I resighted it continuously between B+4, I did not establish its status (but had the impression it was flocking). On the 3/15 sighting it was in a flock of "20 WR which had previously not been using this stretch of the Arroyo (there had been no WR here). This suggests that, as I have been led to believe previously, that there is a succession of events as a flock moves into a new foraging area, involving a shift from flocking to territorial spacing. RyLgr was supplanted by a timberd neighbor 4 times while it foraged across a boundary. One of these supplantations ended when the supplanting bird ventured into RyLgr territory, or very near the border, which was followed by extremely erecting by both birds: [illegible] [illegible] [illegible] [illegible] The intimidation began when the other bird, 3/234, flew at RyLgr, landing 10m to their N, squeaking. RyLgr at this point began anching its feathers and tail, throwing its tail while feeding (not down completely? - a graded display?). Then 3/234