Expedition to Tierra del Fuego – February 12, 2004
We knew on arrival that trapping birds would be difficult; it turned out to be much worse than expected. In the last four days we spent much of the night mist netting from dark to just after high tide getting back to camp no earlier than 2:00 a.m. We ran mist nets each night from the 7th through the 10th and made a decent catch on only one night. The trick was to run the nets after the sun fully set and before the moon rose. Conditions improved through the period, but we had to close the nets on the 9th because of winds that peaked at 45 mph. One night was cloudy and relatively still, but we caught only Hudsonian godwits. When the tide rose to the edge of the salicornia we began trying to trap knots with the cannon nets. We set nets on the 8th through the 10th and placed decoys, made by Dick Jessen and the carvers of the Tuckerton Seaport Museum, near the catch area to attract roosting birds. The problem was predicting the point on the shore where the tide would crest and each day we missed it — one day the tide simply did not come in because of the strong on-shore winds. Knots and godwits always stay at the edge of the tide and would not even the decoys could lure them closer to the net.
But we did succeed on the 9th with a catch of 112 red knots. After all the trapping we caught another 12. We had hoped to catch over 200 birds, but our smaller catch was enough to collect most of the data we sought. From each bird we took measurements, feathers for DNA analysis and cloacal swaps for disease investigations. We caught one bird each banded in Brazil, Argentina and Chile. We caught three birds banded on Delaware Bay, one with an engraved lime-green leg flag banded last year. These flags, engraved with two alphanumeric characters, are an innovation that allows us to uniquely mark individual birds and identify them by re-sighting with a spotting scope. Re-sightings of individually-marked knots enables biologists to estimate adult survival rates.
Carmen Espoz, Aleandra Ponce and Antonio Larrea helped us during banding but spent all day collecting sediment samples to determine the distribution and abundance of marine invertebrate in the major tidal zones of Bahia Lomas. Carmen determined from last years work that the flats of Bahia Lomas can be divided into five major zones, related by their distance from the low tide line. The further from the low tide line than more infrequently flooded the flat. But the flat also subdivides by the speed of the current, which is often determined by the distance from one of the many tidal creeks that dissect the flats.
Last year Carmen sampled along transects (or long lines perpendicular to the coast) and made some interesting discoveries that are important to the birds. There are three major groups of invertebrates, bivalves (small clams), polychetes (small worms), and amphipods. Bivalves occur in greatest numbers starting at the low tide line. As bivalve numbers start to decline, polychetes increase. Amphipods are among the only species living at that part of the flat that is flooded during the highest tides of the full and new moon. Our preliminary data indicated that knots feed on clams and godwits feed on polychetes.
The distribution of birds mirrors the distribution of prey. At the low tide all the birds feed along the low tide line far out from the edge of the salicornia marsh, then either move in with the tied or feed along the tide creek edges. As the tide reaches the infrequently-flooded parts of the flats, birds move to the southern part of the bay where they can feed along tidal creek edges. At the point where all intertidal flats are flooded and the tide gently inundates the relatively prey poor areas of the upper flat, birds start to gather in the areas where flats rarely flood. It’s then that we see some of the biggest flocks red knots and hudsonian godwits.
Prior to one of the nights of mist netting our entire crew went to Cerro Sambrerra to present our projects and results to the people of the town. In the school gymnasium, Carmen and I spoke of our work to a crowd composed mostly of teenagers. Finally, Ricardo Olea Celsi, the provincial mayor of the region, spoke of his gratitude for our work and our dedication to the community surrounding the bay.
Kim Korth 2/12/04